Abstract The cyanidation of argentian plumbojarosite in alkaline medium is a two-step process in series: a slow step of alkaline decomposition, followed by a fast step of complexation of the silver from the decomposition solids. The removal of sulphate ions from the lattice and the formation of an amorphous mixture of lead and iron hydroxides takes place during the alkaline decomposition. The kinetic curves show an induction and a conversion period. The induction period is independent of the initial surface and degree of carbonation of the alkaline medium and exhibits values essentially identical to those observed in argentojarosite. The conversion period is controlled by the chemical reaction of the individual crystals. In NaOH medium the process is first order with respect to [OH −] with an activation energy of 53 kJ/mol. In Ca(OH) 2 medium a substantial reduction of the rate constants is found at a pH near the Ca(OH) 2 saturation point.