An electrosurgical system comprising: an electrosurgical instrument (10, 70) for sealing tissue, the electrosurgical instrument comprising: an end effector (100, 700) having opposing jaw members (110, 120), each jaw member including an electrode (112, 122) having a dielectric coating (113, 123); and the end effector including at least one temperature sensor wherein the at least one temperature sensor senses a temperature of the tissue to be sealed, and characterised by: an electrosurgical generator connected and configured such that: the jaw members transfer energy at a Debye resonance frequency of a manually or automatically selected tissue type through tissue held between the jaw members to effect a seal, wherein said frequency is determined by accessing a Debye resonant frequency versus temperature curve from a look-up table stored on said generator; the at least one temperature sensor provides a temperature feedback signal to the electrosurgical generator, and the generator uses the temperature feedback signal to determine a shift in the Debye resonance frequency of the selected tissue type by accesing said look-up table. an electrosurgical instrument (10, 70) for sealing tissue, the electrosurgical instrument comprising: an end effector (100, 700) having opposing jaw members (110, 120), each jaw member including an electrode (112, 122) having a dielectric coating (113, 123); and the end effector including at least one temperature sensor wherein the at least one temperature sensor senses a temperature of the tissue to be sealed, and characterised by: an electrosurgical generator connected and configured such that: the jaw members transfer energy at a Debye resonance frequency of a manually or automatically selected tissue type through tissue held between the jaw members to effect a seal, wherein said frequency is determined by accessing a Debye resonant frequency versus temperature curve from a look-up table stored on said generator; the at least one temperature sensor provides a temperature feedback signal to the electrosurgical generator, and the generator uses the temperature feedback signal to determine a shift in the Debye resonance frequency of the selected tissue type by accesing said look-up table. an end effector (100, 700) having opposing jaw members (110, 120), each jaw member including an electrode (112, 122) having a dielectric coating (113, 123); and the end effector including at least one temperature sensor wherein the at least one temperature sensor senses a temperature of the tissue to be sealed, and characterised by: an electrosurgical generator connected and configured such that: the jaw members transfer energy at a Debye resonance frequency of a manually or automatically selected tissue type through tissue held between the jaw members to effect a seal, wherein said frequency is determined by accessing a Debye resonant frequency versus temperature curve from a look-up table stored on said generator; the at least one temperature sensor provides a temperature feedback signal to the electrosurgical generator, and the generator uses the temperature feedback signal to determine a shift in the Debye resonance frequency of the selected tissue type by accesing said look-up table. an electrosurgical generator connected and configured such that: the jaw members transfer energy at a Debye resonance frequency of a manually or automatically selected tissue type through tissue held between the jaw members to effect a seal, wherein said frequency is determined by accessing a Debye resonant frequency versus temperature curve from a look-up table stored on said generator; the at least one temperature sensor provides a temperature feedback signal to the electrosurgical generator, and the generator uses the temperature feedback signal to determine a shift in the Debye resonance frequency of the selected tissue type by accesing said look-up table. the jaw members transfer energy at a Debye resonance frequency of a manually or automatically selected tissue type through tissue held between the jaw members to effect a seal, wherein said frequency is determined by accessing a Debye resonant frequency versus temperature curve from a look-up table stored on said generator; the at least one temperature sensor provides a temperature feedback signal to the electrosurgical generator, and the generator uses the temperature feedback signal to determine a shift in the Debye resonance frequency of the selected tissue type by accesing said look-up table.