A three-dimensional (3D) detailed heat transfer model describing the effective thermal behaviour and fluid dynamics of a horizontal metalorganic vapour-phase epitaxy (MOVPE) reactor chamber is reported. Both H-2 conduction and convection were considered, along with the presence of a cooling H-2 flow outside the main MOVPE chamber, the quartz heat conduction through the chamber walls and the convective heat transfer to the ambient air. Computational fluid-dynamic (CFD) simulations indicated that relatively large gas temperature gradients occur normal to the wafer surface both in the process and cooling gas. In the former a fully developed thermal profile is observed. Reduced temperature gradients occur instead in the lateral directions within the process gas flow, giving rise to a good lateral uniformity of the temperature field close to the susceptor. However, large gradients build up in the cooling gas as a result of heat transfer to the surrounding ambient by the free convection. Correspondingly. large buoyancy appears in the cooling flow. A laminar behaviour is instead obtained for the process flow, although the substrate rotation introduces an asymmetry of the gas pathlines above the wafer. Comparing simulated susceptor-ceiling temperature differences (AT) with experimental ones indicates that a free convection parameter alpha approximate to 7.5 W/m(2) K occurs for our system. This is consistent with Ra similar to 10(5) for the ambient outside the MOVPE chamber, suggesting subcritical-free convection conditions. CFD simulations performed as a function of susceptor temperature show close agreement between simulated and experimental DeltaT, indicating the usefulness of a 3D detailed heat transfer approach for the correct simulation of temperature fields in a horizontal MOVPE reactor.