The electrochemical properties of thin films of metal phthalocyanines on conducting substrates are investigated. The electrochromism of lutetium diphthalocyanine is discussed and a film electrode model is proposed based on an irreversible charge transfer step. This model accounts for the main concentration and voltammetric sweep-rate dependencies. The lifetime of lutetium diphthalocyanine films under continuous redox reactions in aqueous solution is restricted due to chemical attack by hydroxide ions. The electrochromic effect is displayed by other phthalocyanines; in monophthalocyamines it is associated with oxidation and reduction of the metal atom. Tin diphthalocyanine behaves similarly to lutetium diphthalcyanine, the redox reaction occurring at the phthalocyanine rings. © 1982.