Coexistence curves of (T, n), (T, x), and (T, phi) where n, x, and phi are the refractive index, the mole fraction, and the volume fraction, respectively, for the binary mixtures {xC(6)H(5)NO(2)+(1 - x)CH3 (CH2)(4)CH3} and {xC(6)H(5)NO(2)+(1 - x)CH3 (CH2)(5)CH3} have been determined within about 10 K from the critical temperatures by measurements of n. The critical amplitude B and the critical exponent beta have been deduced, and the values of beta are consistent with the theoretical ones. The experimental results have been analysed to examine the Wegner correction terms and the behaviour of the diameter of the coexistence curves. The coexistence curves have been successfully described by a combination of the Wegner equation and the expression for the diameter. The power-law dependence of critical amplitudes on molar mass of n-alkane has also been discussed.