The terpolymerization of ethylene, styrene, and carbon monoxide was accomplished by two different palladium-based catalysts: a phosphine-based ligand system and a nitrogen-based ligand system. The range of possible compositions and the composition dependence of the properties of the resulting polymers were determined. These polymers were essentially carbon monoxide versions of the ethylene styrene interpolymers recently presented by Dow. A comparison between the two families of polymers is attempted.