The electron band structure of GaxIn1−xP bulk solid solutions was calculated by the local model pseudopotential method taking into account lattice mismatch. The resulting local strain of the lattice was taken into account in calculations of effective mass and deformation potential. The main optical characteristics of GaxIn1−xP alloys can be explained by the presence of internal local strains and antisite defects. In comparison with bulk samples, the concentration dependences of the spectral peaks E1 and E1+Δ1 in pseudomorphic thin films were found to be more sensitive than the fundamental absorption edge E0 to deformations caused by the substrate.