Abstract The behaviour of thiocyanate and cyanate during the precipitation of potassium jarosite was investigated. Thiocyanate strongly complexes dissolved ferric ion and thereby reduces the amount of jarosite precipitated. The composition of the jarosite, however, is virtually independent of the thiocyanate concentration, and < 0.1 wt% SCN is incorporated in the jarosite structure. Some hydrolysis of the SCN occurs at the 98°C precipitation temperature, and the reaction yields minor amounts of elemental sulphur. The presence of cyanate has little effect on either the amount of jarosite formed or its composition; the jarosite consistently contains < 0.1 wt% CNO. Variations in the pH of the solution at constant concentrations of either thiocyanate or cyanate have little effect on the composition of the jarosite, and negligible amounts of thiocyanate or cyanate are incorporated in the jarosite structure regardless of the pH. Of course, the amount of precipitate decreases as the pH decreases. X-ray diffraction analysis showed all the precipitates to be potassium jarosite, and SEM studies indicated that all the products were composed of ∼ 30 μm cauliflower-like aggregates of tiny jarosite crystals.