Using the empirical pseudo-potential method combined with the improved virtual crystal approximation where compositional disorder is included as an effective potential, we report on the results of composition and temperature-dependent optical band gaps, of the ZnSe1-xSx (0 less than or equal to x less than or equal to 1) type alloys. The obtained results compare reasonably well with the available experimental data. The fundamental band gap is found to vary non-linearly with composition, showing a bowing. Similar bowing is also observed from the temperature-dependent fundamental band gap. This bowing decreases with increasing temperature over the range 0-200 K, but it increases gradually on going from 200 to 300 K. The expression E-g(x, T) = 2.828 + 0.45x + 0.53x(2) + T(-3.45x(2) + 3.22x - 3.88) x 10(-4) was obtained from the data can be used to obtain the energy gap as a function of x and T over a large range of these variables. (C) 2002 Elsevier Science B.V. All rights reserved.