A simple experimental technique to measure the axial average gas temperature in a surface-wave-produced and -sustained plasma has been developed. This experimental method is based on the assumption of a viscous laminar gas regime, and evaluates the gas temperature through the changes in the upstream and downstream pressures when the plasma is switched on and off. Two time scales have been found in the heating and cooling processes, a faster one related to the gas, and a slower one related to the vessel. In addition, the axial average gas temperature has been evaluated as a function of the gas pressure and the incident microwave power at the gap.