The operating characteristics of a prototype two-stage microwave electrothermal thruster (MET) have been investigated. The thruster incorporates a novel supersonic energy addition stage. A Laser Induced Fluorescence (LIF) technique was used to make measurements of temperature and velocity in the exhaust plume, with and without the supersonic energy addition. A numerical model was developed to simulate microwave-sustained discharge in the thruster with argon as the propellant gas. The simulation results agreed with the experimental observations of change both in temperature and plume velocity as energy is added to the supersonic stage of the thruster. In general, as a result of supersonic energy addition, a reduction in temperature and exhaust velocity along the centerline of the thruster was observed. It has been concluded that most of the energy added to the supersonic stage was deposited in the laminar boundary layer. It was also found that heat transfer to the thruster walls was much more significant than expected.