Cubic GaN (c-GaN) samples on GaAs (0 0 1) substrates were grown by RF-plasma-assisted molecular beam epitaxy, in which an As4 overpressure was employed for the nucleating layer. Photoreflectance spectra were obtained in the temperature range from 14 to 300 K. Two independent phenomena were noticed. The first one consisted in optical interference features below the c-GaN bandgap, whose origin is a thermo-optical effect: the ultraviolet perturbation beam changes the refractive index of the c-GaN. The second one represents electro-optical phenomena in which two classical band-to-band transitions occur: the first transition for c-GaN layer in which the temperature dependence reveals defects in the film attributed to a hexagonal fraction estimated previously between 3% and 10%, and a second transition for the GaAs substrate that shows Franz–Keldysh oscillations.