Co-rich CoPt and CoPtP alloy thin films were obtained by electrodeposition. Addition of a P compound to the electrolyte for CoPt deposition induces P codeposition and improves film hard magnetic properties, reaching coercive forces up to 5500 Oe, with S ^∗ = 0.7 . To understand the effect of P compound addition, a macroscopic characterization of intergrain interactions in CoPt and CoPtP films of similar composition and structure has been made by the δM method, using longitudinal Kerr effect as magnetization probe. δM curves reveal exchange coupling in CoPt films, while CoPtP films show mainly magnetostatic interactions. The creation of single-domain grains with nonmagnetic boundaries is proposed as an explanation for the inhibition of exchange interactions in CoPtP films.