Measurements of several fundamental parameters in a microwave-induced atmospheric-pressure flowing plasma are presented. Optical and electrical measurements were performed on argon and argon/nitrogen plasmas in the region 1-7 cm outside the cavity, as the applied microwave power, and plasma composition were varied. The stability of the plasma, atomic emission from the argon support gas, and emission from the analyte species, are proportional to the electron density. The observed electron density was varied when the power was changed,-when an electrophilic species was added, and as the observation zone was moved relative to the microwave field. In all cases, the change in the emission signal followed the change in electron density. The electron temperature, as measured by the double-probe method, is related to the kinetic energy of the fastest-moving electrons in the plasma. It is unchanged by variations in power, plasma gas composition, flow rate, and is independent of the location of the probes relative to the cavity. The spectroscopic and electrical data are consistent with excitation by ion—electron radiative recombination.