An electrosurgical instrument which includes opposing end effectors and a handle for effecting movement of the end effectors relative to one another. The instrument includes a housing and a pair of electrodes. Each electode preferably includes an electrically conductive surface (e.g., which can be dimensioned for sealing, clamping and/or cutting) and an insulating substrate which is dimensioned to be engageable with the end effectors such that the electrodes reside in opposing relation relative to one another. The dimensions of the insulating substrate are different from the dimensions of the electrically conductive surface to reduce thermal spread to adjacent tissue structures. The insulating substrate is made from a material having a Comparative Tracking Index of about 300 volts to about 600 volts to reduce the incidence of flashover.