A new calorimeter has been designed to measure isosteric heats of adsorption from gaseous mixtures with simultaneous measurements of loading and composition. The combined measurements at a single reference temperature provide complete thermodynamic information on the equilibrium behavior of a binary mixture. The components of a binary mixture are dosed alternately so that the individual isosteric heats can be determined from two successive measurements. The necessity of establishing equilibrium within 30 min of adding a dose of gas placed stringent limitations on the design of the sample cell. The attainment of equilibrium was verified by reversing the order of contact of the components of the mixture. The composition of the equilibrium gas phase is determined with a leak valve to a mass spectrometer in order to minimize perturbations of the system.