We examine and compare the diagonal magnetoresistance, Rxx, and the photovoltage induced by microwave (42≤f<300 GHz) and terahertz (f≥300 GHz) photoexcitation in the high mobility quasi-two-dimensional GaAs/AlGaAs system. The data demonstrate strong radiation-induced magnetoresistance oscillations in Rxx to 360 GHz. In addition, cyclotron resonance is observed in the photovoltage to 725 GHz. These results show that our high-mobility GaAs/AlGaAs two-dimensional electron system (2DES) specimens remain photoactive in magnetotransport into the terahertz band. © 2013 American Physical Society.