The pulse radiolysis of deaerated hydroquinone, (H2Q), solutions has been investigated. It has been shown that OH adds to H2Q to form the trihy­droxycyc 1 ohexadi enyl radi ca 1 (H2Q-OH) to H2Q to form the tri hydroxycyc 1 0­hexadienyl radical (H2Q-OH) with k = 5.25 x 109 M-1 5-1. This radical decays by first order kinetics giving the semiquinone radical (HQ). This reaction is catalysed by H . The spectrum of HQ. and its extinction coefficient were  obtained. The decay of HQ is second order and k = 1.2 x 109 M-1 5-1. It is concluded that HQ disappears mainly through reaction with H2Q-H.