Are IOUs part of the petty cash fund?

That all depends on who you owe the money to. Petty cash is basically a small amount of cash that can be used by employees to buy miscellaneous goods such as girl scout cookies or that cleaner people always come to your office to try and sell. Once the money is taken, they have to deposit a slip of paper telling how much money was spent by who, and what did that money purchase. If you buy something small for the business or other employees with the money from petty cash, that IOU is part of petty cash. However, if you have an IOU (I Owe You) towards another business, it is considered to be an Account Payable because you purchased goods with money that you promise them within the next 30 or so days.