The invention discloses a method of heating a preform (50) of thermoplastic material prior to blow molding the same to form a container, with the preform (50) being held at a threaded region (58) during heating thereof. A neck portion (48) of the preform (50) close to the threaded region (58) is temporarily heated more strongly as compared to other regions. The invention further relates to a tempering device for heating such a preform (50), including at least one radiation and/or heat source acting on the preform (50) and defining several impact regions on the preform (50) with different heating intensities each. The preform (50) has associated therewith at least one reflector (52) disposed in the threaded (58) and/or neck region (48) for deflecting radiation proportions piercing the neck region (48) and for heating the neck region (48) on an inside of the preform (50).