The effects of applied magnetic fields on the optical absorption spectra associated with transitions from the Landau valence magnetic levels to shallow donor states in GaAs - (Ga, Al)As quantum wells are studied for magnetic fields applied perpendicular to the heterostructure interfaces. The donor-related magneto-absorption spectra are calculated within the effective-mass approximation, and with a minimization procedure for evaluating donor energies and envelope wave functions. We consider a homogeneous donor distribution in the well and analyse the theoretical impurity-related magneto-absorption spectra for various quantum well widths and applied magnetic fields. The magneto-absorption lineshapes present features clearly associated with transitions involving donor states and different valence Landau magnetic levels.