The heat capacity of the liquid-liquid mixture perfluoroheptane and 2,2,4-trimethylpentane (also known as iso-octane) has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. The theoretical expression for the heat capacity near the critical point was applied to our combined data runs. The critical exponent alpha was determined to be 0.106 +/-0.026, which agreed with theoretical predictions. When alpha was fixed at its theoretical value of 0.11, our value for the amplitude ratio A(+)/A(-) = 0.59 +/-0.05 was consistent with experimental determinations and theoretical predictions. However, the two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, was violated in this system when using the published value for the correlation length. (C) 2001 American Institute of Physics.