In this experimental study, the effect of inducting small quantities of hydrogen on the performance of a natural-gas-fuelled SI engine operating with very lean fuel-air mixtures was investigated. Tests were conducted at a compression ratio of 12.5 under full throttle conditions. Results indicated the possibility of achieving a significant increase in brake thermal efficiency and reduction in HC and NO by addition of hydrogen because very lean mixtures can be used. It was found that hydrogen addition of the order of 24% is needed to achieve a significant improvement in performance. In general, an increase in the quantity of hydrogen added to natural gas decreases hydrocarbon emission levels, extends the lean misfire limit, decreases ignition delay and combustion duration and leads to higher combustion rates. Hence, it is concluded that adding hydrogen to natural gas is a good method to improve performance and reduce emissions when very lean fuel-air mixtures are used.