In this paper the methane-gasoline dual fuel combustion was investigated. Gasoline was injected in the intake manifold (PFI fuel), while methane was injected in the combustion chamber (DI fuel), in order to reproduce a stratified combustion. The combustion process and the related engine performance and pollutant emissions were analyzed.The measurements were carried out in an optically accessible small single-cylinder four-stroke engine. It was equipped with the cylinder head of a commercial 250 cc engine representative of the most popular two-wheel vehicles in Europe. Optical measurements were performed to analyze the combustion process with high spatial and temporal resolution. In particular, optical techniques based on 2D-digital imaging were used to follow the flame front propagation and the soot and temperature concentration in the combustion chamber. The regulated and unregulated emissions were characterized by means of gaseous analyzers, a gas chromatography and an opacimeter at the exhaust. The measurements were performed under steady state conditions at fixed engine speed at different dual fuel ratios. The measurement showed that dual fuel operation permit to improve the quality of the gaseous fuel combustion adding the liquid fuel and the gas and particulate matter emissions are significantly lower compared to the respective ones under gasoline operation.