Dramatic changes of the electronic band structure occur when incorporating even a small fraction of N into GaAs. One important consequence of the N-incorporation is a strong non-parabolicity of the conduction band of GaNxAs1-x yielding already for x less than 1% a considerable increase in the electron effective mass and a strong variation of the electron effective mass with increasing k-vector. We demonstrate how this N-induced non-parabolic dispersion of the conduction band in Ga(N,As) can be determined by a careful analysis of the interband transitions of Ga(N,As)-based quantum wells as a function of hydrostatic pressure. A series of GaN(0.018)AS(0.982)/GaAs wells of various widths was studied by photomodulated reflectance (PR) at 300 K and hydrostatic pressures up to 20 kbar. The PR spectra were fitted using derivative-like line shapes to extract the energy positions of the interband transitions. The transition energies were compared with theoretical values calculated using a 10-band k.p-model including the effects of nitrogen. The good agreement between experiment and theory allows one to extract the valence band offset as well as the conduction band dispersion and hence the change of the effective mass with pressure and energy. (C) 2002 Elsevier Science Ltd. All rights reserved.