Homogeneous polycrystalline InAs1−xPxalloys have been prepared by a halogen vapor‐transport closed‐tube technique over the entire range of composition. Electroreflectance measurements were made at room temperature by the electrolyte method which showed that theE1andE1+&Dgr;1transition energies vary in a concave upwards manner, while theE0′,E2, andE2+&dgr; transition energies vary linearly with composition. Wavelength‐modulated reflectance measurements on semiconducting alloys above the fundamental edge are reported for the first time. These data, taken at room and liquid‐nitrogen temperatures, confirm the interpretation placed on the variation of the electroreflectanceE1peaks. TheE1spin‐orbit splitting (&Dgr;1) remains almost constant in energy asxis increased from InAs and then falls rapidly to the InP value. A similar shape is obtained for &Dgr;1from a k·p calculation for the alloy band structure, which shows that a movement of the transitions in k space is probably responsible for this behavior.