AbstractThe constant‐volume heat capacities of gaseous perfluorocyclobutane and propylene have been measured over a considerable range of temperatures and densities with a new type of adiabatic calorimeter. This calorimeter differs from previous constant‐volume gas calorimeters in having very thin walls and being equipped with an internal motor stirrer to provide temperature uniformity. The experimental results have been compared with the predictions of the Benedict‐Webb‐Rubin and Martin‐Hou equations, by use of published values of the zero‐pressure‐constant‐volume heat capacity. The over‐all agreement is satisfactory, with a maximum difference between the experimental and calculated heat capacities of 6.7%; however the comparison reveals several interesting systematic differences between the experimental and calculated values of the derivatives of the constant‐volume heat capacity with respect to temp