The thermodynamic activities of cobalt in palladium, relative to pure f.c.c. ferromagnetic cobalt, have been determined from e.m.f. measurements on solid-electrolyte oxygen concentration cells over the temperature range SOO-1100ยฐC. The activity of cobalt in palladium exhibits positive departures from ideal solution behavior in cobalt-rich alloys and negative departures in palladium-rich alloys. The activities of palladium, deduced from the experimental data, indicate negative departures from ideality at all compositions. Cobalt-palladium alloys were of particular interest because their unusually high Curie temperatures permitted compositions up to 40 at. % palladium to be studied in the ferromagnetic state. The experimental data were of sufficient precision that the ferromagnetic-paramagnetic transition could be detected in alloys containing nominally 20 and 30 at. '4 palladium. The relative integral molar excess entropies, determined from the changes in the partial molar free energies of mixing as a function of temperature, are positive for all the compositions studied. The heats of mixing are endothermic for cobalt-rich compositions and exothermic for palladium-rich compositions. An extrapolation of the experimental values of the partial molar free energies of cobalt to lower temperatures, suggests the presence of a at high du cobalt le palladium, au cobalt z% partir de f.e.m. B oxygene zt et 1100ยฐC. du cobalt le palladium au comoortement de en cobalt, et nbgatifs en palladium. du palladium, & la un in&& de Curie il est B l'&at ferromag&tique, des alliages dont les compositions vont jusqu'8 40% at. de la transformation ferromagn&ique-paramagn&ique puisse atre detectbe dans les alliages contenant normalement 20 & 30 de palladium. B partir des variations des Energies libres molaires partielles du en fonction de de melange en cobalt et exothermiques en palladium. du cobalt la presence de miscibilite de 600ยฐC en cobalt. in Palladium an festen Sauerstoffkonsentrationszellen im im Palladium in kobaltreichen in palladiumreichen zu 'A Palladium im ferromagnetischen in Legierungen 30 At. 'A Palladium zu erlauben. in kobaltreichen in palladiumreichen zu tiefen auf das Vorhandensein einer Mischungsliicke unterhalb 600ยฐC bei hohen Kobaltkonzentrationen hin.