The aim of the present work is to provide further guidance into better understanding the production mechanisms of soot emissions in Spark-Ignition SI engines fueled with compressed natural gas. In particular, extensive experimental investigations were designed with the aim to isolate the contribution of the fuel from that of lubricant oil to particle emissions. This because the common thought is that particulate emerging from the engine derives mainly from fuel, otherwise the contribute of lubricant oil cannot be neglected or underestimated, especially when the fuel itself produces low levels of soot emissions, such as in the case of premixed natural gas.The fuel-derived contribution was studied by analyzing the influence that natural gas composition has on soot emitted from a single cylinder Spark-Ignition (SI) engine. To achieve this purpose, methane/propane mixtures were realized and injected into the intake manifold of a Single-Cylinder SI engine. The results were compared with pure methane and propane, as well as with natural gas.The lubricant-derived contribution was investigated by injecting lubricant oil either into the intake manifold or directly within the combustion chamber of an optically-accessible version of the engine, requiring no lubrication, in order to mimic the different ways by which lubricant may reach the combustion chamber. The influence on soot emission was assessed in terms of particle number and size distributions. Gaseous emissions and engine performance were also analyzed in order to globally monitor the combustion process.The results indicated that variations in propane content can have strong effects on both performance and emissions. In all tests, natural gas showed the highest PN values. In addition, the results demonstrated that the number of ultrafine particles was very sensitive to the propane fraction at high speeds, because adding propane increased the number of particles between 5 and 30 nm. The effect of feeding the extra lubricant oil was to increase the particles emitted in the lowest range size, independent of the way it was added within the engine.