An unleaded petrol exhaust-gas microwave-induced plasma (MIP) at atmospheric pressure was generated in a TE/sub 101/ resonant-mode cavity. The microwave discharges were generated at three incident microwave power levels: 500 W, 700 W, and 1500 W. An enthalpy probe was used to characterize the exhaust-gas MIP discharge, yielding values of enthalpy, velocity, and heavy-particle temperature along the microwave-discharge axis. The heavy-particle temperature was found to be 710 K, 940 K, and 1065 K, with velocities of 140 m s/sup -1/, 148 m s/sup -1/, and 155 m s/sup -1/, at the exit plane of the discharge tube for an exhaust-gas flow rate of 3.5 l min/sup -1/ at 500 W, 700 W, and 1500 W respectively. The assumption of local thermodynamic equilibrium is required in the calculation of temperature and velocity in the microwave discharge from the enthalpy-probe data. In microwave-induced plasmas, the electron temperature is significantly higher than the heavy-particle temperature. We present arguments that indicate that the calculated heavy-particle temperature and velocity values are nevertheless accurate.