Most mainstream economists would argue they are not. They would say cash and bonds are not economic equivalents. They would argue that issued money wasn’t debt but issued bonds were. A common counter argument would run as follows:
“This idea falls apart at the local gas station… you can’t go shopping, bar-hopping, or on vacation to Miami with a bunch of T-bills and not notice the difference compared to cash…therefore, they are NOT ECONOMIC SUBSTITUTES. Let’s be very clear here: nominal financial *value* DOES NOT EQUAL purchasing power.”
I’ve quoted here from an on-line discussion I’ve just had on this same topic.
The argument on whether bonds are, or can be considered to be money, has perhaps to be considered on several levels. Of course if we need to pay a £9 grocery bill a (genuine) £10 note will be very acceptable to the cashier. A £10 bond, were it to exist in paper form wouldn’t be. Neither of us would know exactly what it was worth at the time. If it was close to maturity it could well be worth something close to £10 but if not, it might not be enough to cover the £9 bill. That’s the only difficulty.
The acceptability argument, though, would also fail €500 notes in Europe. They aren’t easy to spend! No-one wants them. Does that mean they aren’t cash or even money? The best way to get rid of them is to take them into a bank where you have an account and pay them in. The cashier will probably take a note of the serial number in case there is any problem. In that respect a €500 note is no more useful than a paper bond. We’d be able to pay that into our bank account too.
To answer the question we need to consider what money is and why it has a value. To fit in with the concept of financial sector balances, money has to be classed as a liability or a debt to the issuer and an asset to the holder.
So, if we have a closed system of say 100 currency users and their ‘government’ issues 10 currency units to each, the government then has a fiancial debt/liability of 1000 currency units and the users have total assets of 1000 currency units.
The users later switch half their assets to bonds. Say they each buy, at auction, 5 cu of bonds from their government (on the promise that they will be worth, say, 10 cu at some future time) their total assets are still 1000 cu as they can trade the bonds for exactly what they paid for them between each other or even sell them back to the government, if it should have a policy of QE!
The government still has total liabilities of 1000 cu. The fact that it created new IOUs (bonds) and swapped them for its previously issued IOUs (Cash) is of little signficance. If the effective interest rate on those bonds is zero, then zero significance.
The commonly held view that cash and bonds are somehow fundamentally different probably is a hangover from gold standard days. If the government had a policy of backing its cash IOUs but not its bond IOUs with gold, it would make some significant difference. If gold held in bank reserves by government was counted as a financial asset, then issued money wouldn’t represent any net debt, providing it was always on a strict 1:1 basis.
Of course it is arguable that gold is just a commodity like all the other commodities owned by the government and shouldn’t be counted. But if the mindset is that MONEY=GOLD it’s possible to see the logic of the alternative argument.
Modern Monetary Theory: Real Economics 
It’s the same with £50 notes tbh. You’d struggle to pay for your petrol with them.
And of course if you take your deposit account pass book from your building society into the petrol station, they won’t accept that either. Does that mean that doesn’t contain any money either?
Or the cash card from your basic bank account that you have because you are poor and have your benefits paid into it.
Both of those could have more money in them today (because you received interest) or less (because you got charged for an unauthorised overdraft).
Having to take a step to transform what you hold as money into something you can settle a debt with is a standard process. And as long as the process is fully liquid (an ATM cash machine, or holding a bond where there are statutory market makers that have to bid for your bond), then I would argue you have ‘money’.
What stops people spending is perceived loss of liquidity – being asset rich and cash poor. Modern financial processes are all about increasing liquidity so that more and more assets result in actual spending power. You could argue that ebay and cash converters have turned garage junk into money.
Peter-
Let us not forget that other time deposits are included prominently in the money supply aggregates, M2 or M3 (depending on which country) include 6- mo CDs at private banks. CDs are even less “liquid” then T-bills, but the mainstream considers them “money”. Why on earth would my $1000 in my checking account be considered to have “disappeared” when I put it into a 6-mo savings account at the national public bank but not when I put it into a 6 mo savings account at some random commercial bank? It makes no sense whatsoever.
What’s preventing you from “going shopping, bar-hopping, or on vacation to Miami” with your bank credit created out of thin air, backed by a bunch of T-bills?