This thesis investigates how current hybrid bus technology can be improved, through the application of a waste heat recovery system capable of operating on a hybrid bus. At present modem internal combustion engines reject the majority of fuel energy consumed as waste heat through the engine coolant and exhaust streams. By employing innovative technology it is proposed that this, otherwise wasted, heat can be captured and converted to power or to provide useful heating or cooling on the hybrid bus. This recaptured heat will therefore allow an improvement in fuel consumption and a reduction in exhaust emissions. The research carried out in this thesis attempts to determine if the fuel economy of a hybrid bus can be improved by using a waste heat recovery system. The work also aims to discover which type of system is the most suitable for installation on such a vehicle. To achieve this, the modeling of system performance, as well as the design and testing of a fully operational waste heat recovery system on a hybrid bus is presented. The result of the system modeling is benchmarked against the actual, installed system, experimental results which were conducted at M ill brook proving ground U. K. A novel model of an expander was also developed during the research. This model was compared to test data obtained during experimental testing of an expander at Queen's University Belfast. The purpose of the model was to gain a greater understanding of expander leakage and performance while operating in a waste heat recovery system.