Coexistence curves for {x(CHCOOC2H5)(2) + (1-x)CH3(CH2)(6)CH3} have been determined by measurements of the refractive index. The critical amplitude B and the critical exponent beta have been deduced from (T, n), (T, x), and (T, phi) (where n, x, and phi are refractive index, mole fraction, and volume fraction, respectively) curves and the last is consistent with the theoretical values. The experimental results have also been analysed to examine the 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 combining the Wegner equation and the expression for the diameter.