Abstract The autoreduction of ferric porphyrins in the presence of cyanide ions in dimethyl sulfoxide (DMSO) is followed by visible spectroscopy. The presence of an isosbestic point during autoreduction from iron(III) to iron(II) porphyrins suggests it to be an equilibrium process. The observed rate of reduction was found to be linearly dependent on the concentration of cyanide ion. The rate constants calculated for three iron porphyrins viz. tetraphenyl- porphyrin iron(III), deuteroporphyrin dimethyl ester iron(III) and octaethyl porphyrin iron(III) are 240, 165 and 120 M −1 s −1, respectively. The rate constants are found to increase with a decrease in the basicity of the porphyrin. Based on these observations, a nucleophilic attack mechanism is proposed which explains the linear dependence of the observed rate.