The electron effective masses in four epitaxial samples of In1−xGaxAsyP1−ylattice matched to InP (x?0.45y) have been measured by fitting temperature dependent Shubnikov‐de Haas data to the appropriate theory. Fory=0.764, 0.705, 0.756, and 0.467 we have obtained band‐edge effective masses of 0.0464, 0.0471, 0.0478, and 0.0639me, respectively. The compositions were obtained from the band gap using an empirical relation due to Nahoryetal. The method has been verified by applying it to bulk and epitaxial GaAs.