Electron transfer from GaAs wells (Gamma wavefunctions) to AlAs barriers (X wavefunctions) in GaAs / AlAs type II superlattices has been studied by time resolved luminescence with femtosecond resolution. Transfer times can be as short as 140 fs for narrow wells and as long as 25 ps for the widest wells. They do not depend appreciably on the barrier thickness. We discuss the possible contribution to the transfer mechanism of state mixing, intervalley scattering and Frolich interaction. Our measurement favors phonon assisted, intervalley type scattering, modified by the overlap of the different wavefunctions.