Calcium carbonate has been modified with three surface modifying systems based upon stearic acid, acrylic acid and a reactive coating comprising acrylic acid with dicumyl peroxide through dry blending. Polypropylene homopolymer was compounded with the coated and uncoated fillers (0 to 65 wt%) through twin screw extrusion. Infrared and extraction investigations showed that the acrylic acid coatings reacted extensively with the filler surface to produce both tightly bound and loosely bound calcium acrylate. Direct evidence for transfer grafting between the calcium acrylate and the polypropylene in the reactive coating was obtained through selective dissolution of the matrix phase and filler with hot xylene and dilute hydrochloric acid, respectively. For coatings without peroxide, complete dissolution of the compound into the inorganic and organic phases was possible, whilst for those with peroxide, an insoluble fraction was obtained. infrared analysis showed that this was a grafted structure comprising calcium carbonate and polymer. For the filler coating containing acrylic acid with peroxide it was found that the amount of bound polymer at the filler surface was higher for compounds with larger filler loadings indicating a commensurate increase in the extent of the reaction at the filler surface for compounds with higher filler loadings.