Pultrusion is one of the most cost-effective and least laborious techniques of producing structural composite materials. Detailed computer modelling is required for the improved manufacturing and advancement of pultruded composites. This research involves a numerical three-dimensional investigation of the thermochemical aspects of the design and manufacture of pultruded composite materials of various geometries by investigating axially, and in cross-sectional contour form, the transient temperature and curing characteristics for graphite/epoxy composites. The behaviour of the exothermic chemical reaction on the temperature profiles and the degree of cure is discussed. A numerical model based on Patankar's finite difference technique was formulated for solving the governing energy and species equations used in modelling the entire heating section of the industrial PTI Pulstar 804 pultrusion machine. The chemical kinetic parameters of the Epon 9420 epoxy resin were determined using a differential scanning calorimeter. Computer modelling has the ability of enhancing the understanding of the thermal and curing mechanisms without disrupting the performance of composites and can act as a useful tool to improve manufacturing. The cost-effective nature of pre-fabrication modelling allows manufacturers to be more competitive in producing structural shaped composites.