The kinetics of the heterogeneous reaction between dissolved sodium sulfide and biologically produced sulfur particles has been studied by measuring the formation of polysulfide ions, S-x(2-) in time (pH = 8.0, T = 30-50degreesC). Detailed knowledge of this reaction is essential to understand its effect on a biotechnological hydrogen sulfide removal process. The data were fitted with a reaction rate model in which heterogeneous reaction kinetics, decreasing particle size, and a nonuniform particle size distribution were incorporated. Polysulfide ions formed in this reaction have an autocatalytic effect. The observed reaction rate of the autocatalyzed reaction is limited by chemical reaction, contrary to earlier reports for the reaction of sulfide with "inorganic" granular sulfur, which was diffusion-rate-limited. The small particle size or the specific hydrophilic surface properties probably make the surface of the biologically produced sulfur particles more easily available for reaction than the surface of granular sulfur.