n the presence of antioxidants and while protected against UV light, the unsubstituted folates are protected against oxidation. Depending on the combination and concentration of antioxidants, pH, time, and temperature, the oxidation of H4folates is either fully or partly prevented. Antioxidants decrease the rate of oxidative degradation, making it possible to detect even early biologically active oxidation products of H4folate, such as H2folate and folic acid. Hence, a mixture of these forms appears together with pABG and various pterins in foods during processing, cooking, storage, or analytical conditions