We describe a set of measurements which allows for correction of the measured room temperature two dimensional (2-D) electron gas Hall mobility in (Al, Ga)As–GaAs modulation doped structures to account for parallel conduction effects in the doped (Al, Ga)As layer. The measurements confirm that the actual values of the 2-D electron gas mobility are very close to 9200 cm2/Vs for samples with a variety of apparent Hall mobilities and undoped spacer layer thicknesses. This demonstrates that the room temperature mobility is nearly independent of the 2-D electron gas concentration ns when ns<1012 cm-2.