The results of a study on the properties of glass fibres after thermal conditioning at typical engineering thermoplastic processing temperatures are presented. The mechanical performance of rovings and single fibres of well-defined silane-and water-sized E-glass fibre samples was investigated at room temperature after thermal conditioning at temperatures up to 400 degrees C. Thermal conditioning for 15 min led to strength degradation of >50 % at higher temperatures. The tensile strength of silane-coated fibres was relatively stable up to 300 degrees C but exhibited a precipitous drop at higher conditioning temperatures. The water-sized fibres exhibited an approximately linear decrease in strength with increasing conditioning temperature. The strength distribution of the water-sized fibres could be well represented by a unimodal three-parameter Weibull distribution. The strength distributions of the sized fibres were more complicated and required the use of a bimodal Weibull distribution. The results are discussed in terms of the changes in surface coating and bulk glass structure during heat conditioning.