In the course of graft polymerization of styrene on a calcium carbonate powder surface treated with a silance coupling agent such as γ-methacryloxypropyltrimethoxysilane, gelation of the reaction mixture was observed just as on silane-modified clay. Infrared spectroscopic and gel-permeation chromatographic studies have revealed that the extracts of styrene monomer from the powder surfaces contained polymeric silane coupling agent, having crosslinking ability, besides monomeric silane coupling agent. The catalytic effect on the condensation of the silane coupling agent on the powder surface was found to be increased in following order: CaCO3 < P-CaCO3 < clay, where P-CaCO3 is a calcium carbonate modified with phosphoric acid. The order was parallel to the gelation tendency and to the acidity order (H0 ⩽ 6.8) on the surfaces. Direct evidence for bonding between the silane coupling agent and the phosphate moiety on calcium carbonate was not available; however, the silane-modified P-CaCO3 filler exhibited more reinforcing effects in peroxide-cured ethylene–propylene–diene rubber (EPDM) and in sulfur-cured styrene–butadiene rubber (SBR) than the silane-modified calcium carbonate. These reinforcing effects are also discussed in relation to the molecular weight distribution of the silane coupling agent extracted.