The effective rate of gas-solid reactions does not only depend on the chemical reaction rate, but also on the transport of the active compounds through the surrounding gas phase and through the pores of the solid sample. The gas phase transport strongly depends on the shape of the sample holder. Therefore TG measurements of gas-solid reactions show a great range in measured rate and apparent kinetic mechanism, depending on the specific sample holder used. Investigations have been made of the absorption of sulfur oxide onto spherical CaO/Al2O3 pellets. A shrinking-core model was applied for the description of the reaction. So the rate constants of the separate processes can be calculated. In this way the effect of any specific type of sample holder can be quantitatively determined.