Thermocouples have been used to monitor the time versus temperature rise response at the centre of four sizes of pultruded GRP plate used in bolted tension joints in order to demonstrate that all of the GRP plate is at the required temperature prior to testing the joints to failure. A simple one-dimensional finite element (FE) model has also been developed and used to predict time versus temperature rise response. The experimental and numerical results confirm that a thermal preconditioning period of 30 min duration is more than adequate for all of the pultruded GRP plates to achieve the actual test temperatures of 40, 60 and 80 °C.