Phosphate coupling agents, i.e., alkyl dihydrogenphosphates containing functional groups such as olefinic, chloro, methacryloxy, and mercapto, were synthesized and allowed to react with calcium carbonate filler suspended in an aqueous medium. The modification provided the filler with good dispersibility in mineral oil. Physical properties of the vulcanized rubbers loaded with the modified fillers were found to be influenced by the functional group introduced. For example, in peroxidecured ethylene–propylene–diene rubber (EPDM), the methacryloxy group was most effective, and the mercapto group significantly enhanced the physical properties of sulfur-cured styrene–butadiene rubber (SBR), while a saturated aliphatic phosphate was ineffective in both curing systems. Deposition of the coupling agent as calcium salt onto calcium carbonate surface was observed by scanning electron micrography. The novel approach for the surface modification of calcium carbonate filler is descussed in relation to the reinforcing effects on rubbery materials.