Generally, interest is charged on the unpaid balance that has been unpaid for more than one billing cycle - generally 30 days. Additionally, if you take a cash advance on the credit card, not only is the rate higher on that advance, it's also the "LAST" thing your payment is applied to. So if you have a $1000 outstanding balance on your credit card and then take a $300 cash advance, you'll pay (as an example) 14% on the $1000 and 21% on the $300. So if your next minimum payment is $100 and you pay $400, not one penny of that $400 payment goes to paying back the cash advance. You have to specifically and explicitly state that you are paying off the cash advance. This practice has recently come under fire in congress, but it appears to be going nowhere which is no surprise.