We have investigated by Reflection High Energy Electron Diffraction (RHEED) the relaxation of strained Ga1-xInxP layers grown by Gas Source Molecular Beam Epitaxy (GSMBE) on GaAs (100) and GaP (100) substrates. We show that for tensile layers grown on GaAs at 520 degrees C, the 2D-3D growth mode transition occurs around 3-4 monolayers (ML) when the In content reaches 30%. i.e. for a lattice mismatch of 1.4%. This abnormal behavior is nor observed when compressive layers are grown on GaP at 520 degrees C for the same mismatch magnitude. We discuss this difference taking into account the composition and temperature ranges for which spinodal decomposition of Ga1-xInxP alloys have been predicted together with the strain sign.