Dielectric function spectra in the visible-near uv and energy derivative reflectance spectra in the vacuum uv spectral ranges are reported for In1-xGaxAsyP1-y at compositions between y=0 and 1 lattice-matched to InP. Analysis of these spectra yields bowing parameters for the valence-conduction band critical points E1, E1+Δ1, and E'0, estimates of core exciton binding energies, and the ordering of the lower conduction band minima at L and X.