We report a detailed study of piezorellectance (PzR) and contactless electroreflectance (CER) on Ga0.44In0.56NxP1-X epitaxial layers at room temperature. The polarized PzR spectra show anisotropic character along the [110] and [110] directions. Ordering-induced superlattice-like microstructure shown in high resolution transmission electron microscope (HTEM) images confirms the spontaneous ordering in Ga(0.44)ln(0.56)N(x)P(1-x) epitaxial layers. The obtained PzR and CER spectra are fitted with the theoretical line shape functional form. The valence-band maximum, crystal field/strain splitting and spin-orbit splitting to conduction band transition energies denoted as E-g, E-g + Delta(12) and E-g + Delta(13), respectively, are accurately determined. With nitrogen incorporation, the PzR and CER features red-shifts, indicating band gap reduction. The experimentally observed band gap energy reduction of GalnP incorporation nitrogen is of the same order as for nitrogen in binaries. However, the bowing coefficient b is determined to be - 10.5 eV, that is considerable smaller than that of the incorporation of nitrogen in InP (b 16 eV) or GaP (b 14 eV). (C) 2004 Elsevier B.V. All rights reserved.