Coexistence curves for {xC(6)H(5)NO(2) + (1-x)CH3(CH2)(12)CH3} have been determined by measurements of refractive index. The critical amplitude B and the critical exponent beta have been deduced from (T,n), (T,x), and (T,phi) curves, where n, x, and phi are refractive index, mole fraction, and volume fraction, respectively. The values of beta are consistent with the theoretical ones. The experimental results have also been analysed to examine Wegner correction terms and the behaviour of the diameter of the coexistence curve in various units over a temperature range of 10 K from the critical temperature. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter. The molar mass (M)-dependence of critical volume fraction phi(c) may be expressed as (1-phi(c))phi(c) proportional to M-r and -r is found to be 0.40.