An electrosurgical system including an electrode assembly having two electrodes for use immersed in an electrically conductive fluid has a generator with control circuitry for rapidly reducing the delivered radio frequency output power by at least 50% within approximately 100 mu s, corresponding to the time needed for a few cycles of the peak radio frequency output voltage to reach a pre determined threshold limit. In this way, tissue coagulation can be performed in, for example, saline without significant steam generation. The same peak voltage limitation technique is used in a tissue vaporization or cutting mode to limit the size of the steam pocket at the electrodes and to avoid electrode burning.; The output power reduction is abtained by "on time control" (70) by producing a signal synchronous with the RF oscillator, thereby reducing progresseively the "conduction state" of the gate (80) which connects the power supply with the oscillator. Switch mode power supply (66) can itself vary its output voltage as a function of signal (66) from the controller (72), the speed of variation however will be much slower than the one produced by "ON" time control mechanism.