Single crystal samples of CuGa(Se 1- x S x ) 2 alloys have been grown by an iodine transport method and measurements made of the reflectance spectrum of these samples in the temperature range 4–300 K. For each sample the values of energy gap E A and E B corresponding to the two higher valence bands A and B have been determined but the third gap could not be observed. The variation of E A and E B with temperature has been fitted to the equations proposed by Manoogian and Leclerc, i.e. E - E 0 = U T s + V θ(cothθ/2 T -1). The variation of the parameters V and θ as a function of composition are discussed. For the case of CuGaS 2 , E A - E B gives the value crystal field splitting Δ cf and the variation of this parameter with temperature is determined. It is shown that the variation in Δ cf can be explained in terms of a variation with temperature in the contribution of the upper d orbitals to the valence band, the results being in fair agreement with the values proposed by Yamamoto et al.