Gas mixtures with high xenon densities are explored for continuous flow spin-exchange optical pumping. It is shown that the Xe-129-NMR signal increases significantly with increasing xenon partial pressures up to about 200 kPa, despite a decreasing spin-polarization. Comparison of the rubidium infrared D-2 emission with the xenon polarization demonstrates that radiation quenching by molecular nitrogen is of no substantial benefit for the pumping process at xenon pressures above 100 kPa. This reflects a diminished importance of spin-depolarization by radiation trapping due to the increased significance of spin-relaxation by rubidium-xenon collisions at high xenon densities. A quantitative expression for this effect is provided. (C) 2003 American Institute of Physics.