Neutron three-axis spectrometry has been used to determine the interplanar magnetic exchange parameter in the magnetic van der Waals compound CoPS3. The exchange is found to be small and antiferromagnetic, estimated to be 0.020 +- 0.001 meV, which is surprising considering that the magnetic structure is correlated ferromagnetically between the ab planes. A possible explanation, involving a small anisotropy in the exchanges, is proposed. The results are discussed with reference to the other members of the transition metal-PS3 compounds.