An electrosurgical generator is disclosed. The generator includes an RF output stage configured to supply electrosurgical energy to tissue via at least one active electrode configured to apply electrosurgical energy to tissue sensing circuitry configured to measure impedance of tissue and a controller. The controller is configured to determine whether a tissue reaction has occurred as a function of a minimum impedance value and a predetermined rise in impedance, wherein tissue reaction corresponds to a boiling point of tissue fluid generate a target impedance trajectory as a function of measured impedance and desired rate of change based on the tissue reaction determination, wherein the target impedance trajectory includes a plurality of target impedance values and drive tissue impedance along the target impedance trajectory by adjusting the output level of the electrosurgical generator to substantially match tissue impedance to a corresponding target impedance value for at least a predetermined minimum time period.
