A dynamic model is derived for simultaneous, homogeneous and heterogeneous, first-order reactions in a slurry reactor. The theory is applied to experimental data for the oxidation (with gaseous oxygen) of aqueous sulfite solutions which is first order in oxygen and zero order in sulfite. The homogeneous reaction is very slow at low pH (2–3) where the sulfur exists as dissolved SO2 or HSO3, even when cobalt is added as a homogeneous catalyst. At higher pH (7–8.5), considerable homogeneous reaction occurs with and without cobalt ions. Here the predominant species are SO3= and HSO3. Activated carbon is not a heterogeneous catalyst at pH 7 or 8.5, but is responsible for nearly all the reaction at the low pH values. Analysis of the heterogeneous data indicates that the surface reaction between adsorbed oxygen and SO2 controls the rate rather than adsorption of oxygen.