The AlxGa1−xP alloys having compositions up tox= 0.74 are prepared by slow cooling from Ga‐rich Al&sngbnd;Ga&sngbnd;P solutions. The composition of the alloy is accuratley determined from the density measured by using the flotation method. The alloy composition almost corresponds to the atomic ratio of Al to P in the initial mixture. The energy gap and the green‐band emission peak of the alloy vary linearly to the higher‐energy side as the Al concentration increases.