What does peroxide value exactly measure?
The test measures the amount of hydroperoxide by a redox reaction with iodide:

$\ce{2ROOH + 2H+ +2I- -&gt; 2ROH + I2 + H2O}$

In the course of the reaction, the hydroperoxide is reduced, while iodide is oxidized to iodine. In principle, this is a variation of the iodine clock reaction of hydrogen peroxide.