Using a high-temperature flow calorimeter, we determined the molar heat capacities at Ž constant pressure C of four binary mixtures at three different temperatures 410.3, 514.7, p, m . Ž and 630.6 K and various compositions. The binaries investigated are: benzene q . Ž . Ž . Ž tetrachloromethane , benzene q trichloromethane , benzene q acetone , and acetone q . trichloromethane . At T s 630.6 K, all these mixtures showed ideal gas behavior, i.e. heat capacity additivity. However, excess heat capacities were found at lower temperatures, Ž . Ž . particularly in Lewis base q acidic proton and non-polar q polar binary systems, such as Ž . Ž . Ž . acetone q trichloromethane , benzene q acetone , and benzene q trichloromethane , but Ž . no significant excess heat capacities were detected in non-polar q non-polar systems. The excess heat capacity can be attributed to intermolecular interaction between unlike molecules. The heat capacities of the binary mixtures considered in the experiment were also calculated Ž . using both cubic van der Waals -type and virial-type equations of state: PR EOS and truncated virial EOS, respectively. The two EOSs gave comparable results. However, there are some statistically significant biases between the calculated results and experimental values in systems containing a polar component. This may be due to the limitations of the mixing rules used in the calculation.