Electroreflectance measurements by the electrolyte technique have been made on alloys of the InAsxSb1−x, InAs1−xPx, and GaxIn1−xSb systems and the variation of the values of E1, E1 + Δ1, E0′, and E2 found as a function of composition x. These results have been analyzed together with those previously published for the GaAs1−xPx and GaxIn1−xAs alloys. It is found that in all five cases, the deviation from linearity of the E versus x curves must be attributed mainly to the effects of disorder in the alloy lattice. For each curve, a parameter measuring the effect of disorder has been determined and the results are discussed in terms of the band structure of the alloys.