In a C02 gas discharge laser energized by a radio frequency (RF) power source a transformer (20) having selectively variable output impedance is used to match output impedance of the power source to the impedance of discharge electrodes of the laser. A similar transformer (40) can be used to impose a selective variable phase -shift on the RF power from the source. The variable impedance transformer can also be used for impedance matching between amplifier stages in the power source. The transformer comprises first (28, 46) and second (32, 48) transmission - line sections connected in series between and the load. Each of the first and second transmission - line sections having an electrical length equal to or less than about one- twelfth of a wavelength at the source- frequency. A selectively variable electrical component (Cv, Lv) is connected to the node between the first and second transmission - line sections to optimize the transfer of RF- power between the source and the load.