This International Standard specifies a method of determining the total active-oxygen content of organic peresters. It applies to those peresters used to initiate crosslinking of unsaturated-polyester thermosets. The peresters most widely used in the crosslinking of unsaturated-polyester resins are given in Table 1. Butyl peresters may contain traces of tert-butyl hydroperoxide (TBHP), the active oxygen in which can be determined separately from that in the perester itself (see annex A). The active-oxygen content of the perester alone can then be obtained by subtraction.