The coexistence curves and the turbidity of(nitrobenzene + undecane) have been measured, and the critical amplitudes and exponents relating to the difference of density variables of two coexisting phases, the correlation length, and the osmotic compressibility have been deduced. The values of exponents are consistent with the theoretical predictions. The experimental results of 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 has been used to describe successfully the coexistence curves.