The dependence of the reactivity in the solid state of powder mixtures upon the manner of mixing has been studied in the case of the reaction : BaCO₃(s) + TiO₂(s) --> BaTiO₃(s) + CO₂(g) The most important parameters affecting the conversion degree are the grain size, the molar ratio of the mixture, and the method of mixing. The authors have developed two models allowing prediction of the variations of the number of contacts between the grains as a function of the parameters studied. Experimental and theoretical results agree.