Refractive indices and absorption coefficients of In1−xGaxP1−yAsyfory= 0 to 1 lattice matched to InP and of GaAs and GaP have been measured by ellipsometry in the wavelength range between 365 and 1100 nm. The high‐purity layers (7×1014<n<2×1016cm−3) used here were grown by liquid phase epitaxy. The quaternary refractive indices were also calculated from the measured indices of the constituent binary compounds by averaging the quantity (&egr;−1)/(&egr;+2). The &egr; values were all taken at the same energy separation from the respective band gaps. The results of this new averaging procedure are compared with the measured indices and with the refractive index steps to InP deduced from lasers. In addition, the absorption coefficient of InP near the band gap was measured as a function of the doping level. Finally, the temporal growth of the natural oxide layer thicknessdoxon various InP and GaAs samples was determined by ellipsometry. For InP, the increase ofdoxis 0.39 nm per decade of time.