Abstract: A dual magnetron sputtering power supply for use with a magnetron sputtering apparatus having at least first and second sputtering cathodes for operation in the dual magnetron sputtering mode, there being a means for supplying a flow of reactive gas to each of said first (1) and second (4) cathodes via first (12) and second (14) flow control valves each associated with a respective one of said first and second cathodes and each adapted to control a flow of reactive gas to the respectively associated cathode, the power supply having, for each of said first and second cathodes a means for deriving a feed-back signal relating to the voltage prevailing at that cathode, a control circuit for controlling the flow of reactive gas to the respectively associated cathode by controlling the respective flow control valve and adapted to adjust the respective flow control valve to obtain a voltage feedback signal from the respective cathode corresponding to a set point value set for that cathode.