Parametric observations on an atmospheric-pressure plasma sustained in ambient air by an argon discharge excited by 2.45 GHz microwaves in an open-ended dielectric discharge tube are reported. Microwave power, discharge tube dimensions, and argon flow rate were the major operating parameters. Three distinctive plasma regions were observed: plasma filaments exiting from the discharge tube, converging point of these filaments, and a plasma flame. At the filament-converging point, argon atom excitation temperature, rotational temperature, and electron density were measured by optical emission spectroscopy (OES) in the operating range of (3.0-5.0) liters per minute of gas flow rate and (650-950) W of microwave power. The measured excitation temperature and rotational temperature were (5000-5800) K and (2800-3400) K, respectively. The electron density obtained by Stark broadening width of the H-beta line showed (5.0-8.0)x10(14) cm(-3). It was observed that the volume of the plasma flame and the gas temperature were increased with increasing the microwave power. On the other hand, higher gas flow rates increased the electron density. In the plasma flame, the gas temperature measured by a thermocouple and OES was in the range of (1030-2200) K, which showed an exponential decrease in the axial direction away from the converging point. (C) 2002 American Institute of Physics.