The oxidation of pyrite (FeS2) has been investigated in a fixed-bed laboratory reactor to gain knowledge about the SO2 formation mechanisms and kinetics at conditions relevant to the upper stages of a cyclone preheater tower in a modern dry kiln system for cement production. Experiments were carried out with a high sulfide containing shale (a mass-average diameter equal to 21 mum) and with pure FeS2 particles (between 32 and 64 mum). Measurable SO2 formation started at about 350 degreesC for the shale and at 400 'C for the FeS2 particles and increased with temperature for both materials. Experiments showed that the conversion of pyritic sulfide to SO2 was independent of the inlet SO2 concentration up to at least 925 ppmv and that the conversion decreased as the O-2 concentration was increased from 5% to 20% (v/v). A shrinking-core reaction mechanism with FeS2 as the core and with porous FeS as the intermediate product layer is proposed to account for the experimental observations. According to this mechanism, the oxidation of the FeS2 core to FeS is relatively fast at moderate O-2 levels (e.g., 5%) because it is easy for O-2 and SO2 to diffuse through the porous product layer of almost non-oxidized FeS. At increased O-2 levels (e.g., 20%), we believe that the FeS layer also starts to oxidize so its porous structure is disrupted whereby the resistance to diffusion through it increases and thus further oxidation of the FeS2 core is inhibited, leading to lower overall conversions.