The excess second virial coefficient of (0.500CO + 0.500CO(2))(g) has been determined at temperatures 303.15 K, 313.14 K, 323.15 K, 333.15 K, and 343.15 K. The pressure-change-of-mixing method was used. The excess molar enthalpy was derived as well as unlike-interaction second virial coefficients which were compared with predictive equations.