We measured the dielectric constants of strained In0.7Ga0.3AsyP1−y(y=0.2, 0.4, 0.8, 1.0) and lattice-matched 1.32 &mgr;m In1−xGaxAsyP1−ythin films grown on InP substrates by metalorganic chemical vapor deposition. Measurements were performed by phase-modulated spectroscopic ellipsometry in the range of 0.76–4.9 eV. Our data bridge the gap between literature data in the near-infrared region and those in the visible-ultraviolet region. The critical point energies of strained In0.7Ga0.3AsyP1−ywere compared with unstrained counterparts and were found to be shifted in accordance with the theory, which predicts that the compositional shift is compensated. Thus, the critical point energies of strained In1−xGaxAsyP1−ythin films of arbitrary composition can be estimated accurately and, conversely, the composition of strained In1−xGaxAsyP1−ythin films can be estimated by measuring their critical point energies, as for unstrained materials. ©1997 American Institute of Physics.