High quality AL x Ga1−x N-GaN-Al x Ga1−x N quantum wells of different thicknesses andx values were grown by low pressure metalorganic chemical vapor deposition (LPMOCVD). The change in their emission energies (measured at 77 K by photoluminescence) as a function of both well width andx value was typical of a type I heterojunction. The experimental data was compared to theoretical calculations based on the finite square well model and the confined particle transitions were identified. The experimentally observed energy shifts differed from calculated values of then = 1 electron to heavy hole transition by a constant amount (for a givenx value) attributed to strain in the AlGaN-GaN system. Also, an estimate of the critical thickness in the AlGaN-GaN system was determined based on the Matthews and Blakeslee force balance model.