Both the heat capacity and the turbidity of the liquid-liquid mixture succinonitrile-water near its upper critical consolute point were measured and two amplitude relations were tested. Using an adiabatic calorimeter to measure the heat capacity and the transmitted light intensity to determine the turbidity, precise and reproducible data determined the critical exponents alpha, nu, and gamma consistent with theoretical predictions. The correlation length xi (0) = 0.168 +/-0.004 nm was determined from the turbidity experiment while the heat capacity amplitudes were A(+) = 0.0543 +/-0.0004 J/(cm(3) K) in the one- and A(-) = 0.1013 +/-0.0004 J/(cm(3) K) in the two-phase region. The amplitude ratio A(+)/A(-) = 0.536 +/-0.005 was consistent with other experimental determinations in liquid-liquid mixtures or liquid-vapor systems, and with recent theoretical predictions. The two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.017 and 0.020, was determined to be 0.0187 +/-0.0013. (C) 2001 American Institute of Physics.