The microwave-induced magnetoresistance oscillations are exhibited by the GaAs/AlGaAs two dimensional electron system (2DES) under microwave and terahertz photo-excitation at liquid helium temperatures. Such oscillations are presently understood in terms of various theories. In order to identify the relative physical contributions, we have concurrently examined magnetotransport and microwave reflection from the 2DES. For the reflection measurements, a sensitive microwave detector was assimilated into the standard experimental setup. Here, we correlate changes in reflection with the concurrent transport response of the photo-excited 2DES.