You pay income tax on the amount of interest that the bond accumulated.
You have the option of declaring the income each year (annoying, because it's up to you to do the research and report the correct amount), or paying the tax in the year you cash in the bond (which is what most folks do).
So if you paid $500 for a bond, and 12 years later you cash it in and get $985, you would report $485 in interest on your tax return.

