The paper is concerned with diffusion controlled reaction mechanisms and the associated rate equations. A review of previously reported rate equations was carried out and reported with the proofs of the equations. Three new rate equations are proposed based on the reaction of a solid and a liquid phase. Isothermal and non-isothermal experimental data was obtained for the reaction of sodium carbonate and calcium silicate and the data analysed to find the rate equation that best fitted the data. Isothermal data was analysed by reduced time plots and non-isothermal data was analysed by Sestak's method. One of the proposed equations provided a more perfect fit to the experimental data than any of the previously known rate equations. While the experimental temperature suggested a solid-solid reaction mechanism, the experimental data gave a best fit with one of the proposed solid-liquid equations. An experimental method was devised to obtain mass loss data by TG on a rising temperature scale for liquid-solid phases without prior interaction in the solid-solid phase. The experimental data from this experiment could only result from a solid-liquid interaction and was shown to fit the same proposed rate equation as was observed in the solid-solid experiments. It was concluded that a liquid phase was produced during the reaction at temperatures lower than might otherwise be expected.