We have demonstrated heterodyne detection up to an intermediate frequency of 26.5 GHz using quantum well infrared photodetectors. A CO2 laser and a lead-salt tunable diode laser were used as the infrared sources, Heterodyne detection experiments measure the high frequency behavior of photoexcited electrons and their transport properties, We have also carried out microwave rectification experiments which measure the high frequency behavior associated with the dark-current electron-transport processes.