We derive an analytical model to describe the conduction-band states of GaNAs-based quantum well structures, including the band anticrossing effect between N resonant states and the conduction-band edge. The predictions of the model are compared to those obtained using a full ten-band k·p model based on the same set of parameters. Both methods are then tested by comparison with the experimentally determined ground- and excited-state interband transition energies of GaN_xAs_1−x quantum wells of different well widths and N composition x obtained at 300 K and under hydrostatic pressures up to 2.0 GPa . We show that the transition energies can be described by a consistent set of material parameters in all the samples studied, and present how the conduction to valence-band offset ratio varies strongly with x in GaN_xAs_1−x∕GaAs quantum well structures. We conclude that the model presented can be used to predict the transition energies and electron subband structure of any GaN_xAs_1−x∕GaAs quantum well with well width between 2 and 25 nm , and N composition x between 1 and 4% , although further work is still required to confirm the optimum choice for the variation of band offset ratio with composition.