Either an endoscopic or open bipolar forceps includes a flexible, generally tubular insulating boot for insulating patient tissue, while not impeding motion of the jaw members. The jaw members are movable from an open to a closed position and the jaw members are connected to a source of electrosurgical energy such that the jaw members are capable of conducting energy through tissue held therebetween to effect a tissue seal. A knife assembly may be included that allows a user to selectively divide tissue upon actuation thereof. The insulating boot may be made from a viscoelastic, elastomeric or flexible material suitable for use with a sterilization process including ethylene oxide. An interior portion of the insulating boot may have at least one mechanically interfacing surface that interfaces with a mechanically interfacing surface formed between the shaft and a jaw member or with a mechanically interfacing surface disposed or formed on the shaft or a jaw member.