We investigate GaNAs/GaAs multiple quantum well structures using the photoreflectance spectroscopy at various temperatures. The modulated optical response consists of quantum well excitonic transitions and band edge transitions that exhibits Franz-Keldysh oscillatory features. The bowing parameter, effective mass, and the band-offset value were adjusted to obtain the subband energies to best fit the observed quantum well transition energies. The period of the Franz-Keldysh oscillations indicates the strength of the internal field.