Coexistence curves of (T, n), (T, x) and (T, phi), where n, x, and phi are refractive index, mole fraction and volume fraction, respectively, for the binary mixtures {xN-methyl-2-pyrrolidone (NMP) + (1 - x)cyclohexane} and {xNMP + (1 - x)cyclooctane} have been determined in the critical regions 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 rectilinear 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 behaviour either on the molar volume of cycloalkane or on the interaction parameter per unit volume has also been discussed. (C) 2003 Elsevier Ltd. All rights reserved.