The rate of sulfur dissolution in aqueous sodium sulfide at various alkalinities was studied in a stirred tank and a flow reactor with fixed bed. The rate was approximately proportional to the specific surface of the sulfur. The mass transfer in the liquor is of minor importance as long as all particles are reached by liquor. Sulfide ions seem to react according to a second-order reaction at a moderate rate, but polysulfide ions, possibly in combination with sulfide ions, react according to a mixed first-order reaction at a high rate. The former reaction has a fairly low and the latter a very low activation energy. Experimental evidence suggests that small polysulfide ions are more active than larger ones.