This paper details a theoretical and experimental study of combustion phenomena within a two-stroke cycle, spark ignition engine. The theoretical part of the work involved the development of an improved quasi-dimensional combustion model. This model was incorporated into a computer program which was used to predict the thermodynamic and chemical changes occurring within a two-stroke engine during the closed cycle of the engine. The simulation uses a turbulent kinetic energy model to predict flame front velocity. Combustion chamber geometry is used to estimate entrained mass and mass fraction burned is calculated from a simple eddy-entrainment approach. The experimental work was undertaken to validate the combustion model. Two separate cylinder heads were designed with different combustion chambers and tested on a standard loop-scavenged engine over a range of operating conditions. This validation showed good correlation between measured and predicted results, but the differences seen highlight some areas requiring further consideration. It is postulated that the main influencing factors are squish effects and cycle-to-cycle variation.