Atom Probe Tomography (APT) was used to obtain a direct evidence of chlor segregation and cobalt diffusion at the grain boundaries (GBs) of polycrystalline coatings deposited by moderate temperature chemical vapor deposition (MT-CVD) on a WC-Co cemented carbide substrate. Reasons behind segregations are discussed, and its effects are correlated to the micromechanical properties of Ti(C,N) and Zr(C,N). It is concluded that chlorine segregation is a relevant factor for explaining the low cohesive strength at the GBs of Ti(C,N) leading to intergranular failure during micro-compression testing, while its absence in Zr(C,N) along with Co diffusion contribute to grain boundary strengthening.