A method for providing an internal surface temperature profile of a thermoplastic preform (10, 22) during a stretch blow molding production process comprising at least the steps of: (a) heating a plurality of in-line cold preforms (10) in the production process to provide a plurality of in-line heated preforms (22); (b) diverting at least one of the in-line heated preforms to provide at least one off-line heated preform (30); (c) providing the remaining the in-line heated preforms to one or more stretch blow molds (24) for forming the heated preforms into blow molded products (12); and (d) locating one or more temperature sensors (34, 36) inside the or each off-line heated preform (30) of step (b) to provide an inside surface temperature profile of said preform(s).