The formation of stearate on precipitated calcium carbonate (PCC) and magnesium hydroxide has been examined. The object of coating the filler surface is to achieve improved mechanical properties in the resulting composite material. The coating of a filler with stearate allows the modification of the energies of interaction so as to improve dispersion and alter the mechanical properties of the interphase region. In this work the use of Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption isotherm analysis, thermal gravimetric analysis (TGA) and carbon–hydrogen–nitrogen combustion analysis (CHN) have been used to characterise the stearate on the surface of the calcium carbonate filler. New methods for the estimation of fractional coverage and coating thickness calculation have been developed. Using dynamic mechanical thermal analysis (DMTA) the effects of the coating on the interphase region of the composite have been demonstrated.