Numerous process parameters involved in different stages of gas carburising make the process difficult to control, which in turn makes predictions of properties difficult. In the present work, a computer program was developed to simulate the entire gas carburising process, focusing on the thermodynamics of the gas phase and thermochemical reaction kinetics at the gas/solid interface in endothermic atmospheres. By combining gas phase kinetics and carbon activities in gas and steel with the alloy dependent diffusivity of carbon in austenite, the program can predict accurate carbon diffusion profiles for both batch and continuous furnaces. The program has been designed to account for all the major process variables that affect resultant carbon diffusion such as steel alloy composition, part geometry, gas composition and basic treatment parameters with their possible variations during the process. It is also able to predict other important properties such as transformation temperatures and the as quenched and tempered hardness of carburised parts. Comparison with the results of the experiments performed in industrial carburising furnaces provided validation of the model predictions.