The coexistence curves and the turbidity of (nitrobenzene + tridecane) have been measured, and the critical amplitudes and exponents relating to the difference in density variables of two coexisting phases, the correlation length, and the osmotic compressibility have been calculated. The values of exponents are consistent with the theoretical predictions. The experimental results of the coexistence curves have also been analysed to examine Wegner correction terms and the behaviour of the diameter of the coexistence curves. A combination of the Wegner equation and the expression for the diameter have been used to successfully describe the coexistence curves. The molar mass dependence of the critical amplitudes of the correlation length and the osmotic compressibility has been discussed.