The magnetic susceptibility, magnetisation, electrical resistivity and thermopower were measured for the compounds (Gd,Y)₄Co₃, which form a complete solid solution. The localised f electrons of Gd exert a molecular field on the itinerant d electrons of Co, causing exchange splitting for the latter above about 70% Gd. Below this concentration the magnetic moment of Co is small, about 0.3 mu _B per formula unit and arises from localised d electrons on certain Co sites. The exchange integral evaluated using the model developed by Bloch et al. (1975) characterising the molecular field varies rapidly for the Y rich compounds reflecting a rapid variation of the susceptibility of Y₄Co₃ with temperature. The electrical data show clearly the profound influence of the itinerant d electrons in causing high-temperature curvatures and of the localised f electrons in causing kinks at the Curie temperatures.