The role of acid-base interactions in the nonreactive surface treatment of mineral fillers was studied by a dissolution method. The effect of treatment on the surface properties of CaCO3 covered with stearic acid was determined by ESCA measurements and by the calculation of the basicity of the treated fillers. The dissolution measurements were carried out in twelve different solvents. The maximum amount of surfactant adsorbed on the filler surface (c(max)) proved to be independent of the acid-base character of the solvent, while the amount bonded proportionally (c(100)) depended strongly on it. The results showed that competitive processes determine the surface coverage of the filler : the adsorption of the surfactant and the solvent on the filler surface competes with the solution of the treating material in the solvent. Sedimentation experiments gave valuable information about the strength of solvent/filler interaction. The surface characteristics of the filler changed with surface coverage; the surfactant concentration at which basicity attains its minimum agrees well with the monolayer coverage determined by ESCA and with the c(100) value obtained in apolar solvents. The results have practical relevance as well, similar competitive solution/adsorption processes may take place also in composites containing surface treated fillers.