We studied the optical polarization of surface-emitted photoluminescence from thick semi-polar (11–22) AlxGa1−xN layers on m-plane sapphire substrates with aluminum contents x between 0.0 and 0.63 at T = 10 K. Luminescence with an electric field vector E parallel to the in-plane direction [1–100] prevails for x < 0.2. Polarization with E parallel to the perpendicular in-plane direction [11-2-3] prevails for x > 0.2. In case of low aluminum content, the spectra are dominated by basal plane stacking fault emission. The degree of optical polarization for both basal plane stacking fault emission and near band edge emission is comparable.