We calculate with the nb initio pseudopotential method the valence-band offset (VBO) of GaAs/GaP lattice-mismatched heterojunctions pseudomorphically grown on GaAsxP1-x(001) substrates (0 less than or equal to x less than or equal to 1). The resulting value of 0.76 eV for the GaP substrate supports the highest among the experimental data available. We show that the VBO for any substrate can be easily obtained from that calculated for one substrate following standard band-structure theory since the interface-dependent contribution to the VBO, which is the electrostatic potential lineup, has no significant variation with the substrate. Such variation, however, can be taken into account accurately by a simple analytic scaling law, which is demonstrated here. Finally, we show that the VBO of GaAs/GaP (001) is due to structural and chemical effects, and we propose an operative method for identifying and separating the two contributions, whose dependence on strain is also discussed.