The effect of high-pressure curing within an autoclave on the cured glass transition temperature (T-g) of a thermoset fibre-reinforced composite has been studied. The results indicate that an increase in the T-g value was obtained by a higher curing pressure as well as a lower moisture content. Both results have been related to the reduction in the microscopic free volume and the macroscopic void content of the matrix resin. Using the results obtained, the T-g increase was related to the applied autoclave pressure through equations based on both free-volume and thermodynamic concepts, with the results indicating a significantly higher effect than for homogeneous polymers. This has been attributed to a moisture-dominant diffusion process which has been used to explain the growth and reduction of voids.