A promising method for the recovery of the pulping chemicals at kraft pulp mills is the direct causticization. In this process, a gasifier and a titanate cycle replace the conventional recovery boiler and lime cycle. The aim of this study was to investigate the influence of carbon dioxide on the reaction kinetics for the solid state reaction between sodium carbonate and sodium trititanate, i.e. the direct causticization reaction occurring in the gasifier in this process. Experiments were carried out at five different temperatures (800-880°C) and with five different amounts of carbon dioxide in the inlet gas (0-5%) in a differential reactor made of quartz glass. Kinetic data was obtained by measuring the release of carbon dioxide during the reaction. Different kinetic models were used to describe the conversion. The Valensi-Carter model describes reactions controlled by diffusion in the solid material and the phase-boundary model describes reactions controlled by chemical kinetics for a first-order reaction. Furthermore, a model including both diffusion in the solid material and chemical kinetics, the "modified shrinking-core" model, was used. It was found that higher temperatures decrease the time to reach complete conversion. This was found for all carbon dioxide concentrations. Differences could also be seen between experiments with and without carbon dioxide, but no clear differences were seen for different amounts of carbon dioxide. The change of controlling reaction mechanism occurred at different temperatures for different amounts of carbon dioxide in the inlet gas. When fitting the models to experimental data in the whole conversion interval it was found that the reaction was controlled by diffusion for all amounts of carbon dioxide at low temperatures. Though, when the carbon dioxide concentration in the reaction atmosphere was increased, the change of reaction mechanism to chemical kinetics occurred at higher temperatures. However, when there was carbon dioxide in the reaction atmosphere none of the models could give a good visual description in the whole conversion interval even if reasonably good standard deviation between the models and the experimental data was obtained. When the phase boundary model was fitted to the experimental data in the conversion range 8-73%, it could describe the data very well both visually and by the standard deviation calculations, which indicate that the reaction is controlled by chemical kinetics in the beginning of the reaction.