The paper describes investigations on the nature of the misfire phenomenon in a SI engine. Authors performed a series of experiments on a single-point gasoline injection engine in a wide range of loads and rotational speeds, including different air-fuel ratios. The data was gathered using optical combustion sensor integrated with a spark plug.The main aim of the experiments was to develop fast and reliable misfire detection algorithm basing on the optical signal, which would be simple and easy to use within the engine control system. Misfire identification algorithm proved to be reliable, with neglectable detection error.Analysis of the results included not only quantitative measurements, but also those regarding distribution and probability of occurrence. Results made it possible to establish conditions in which the investigated engine is more likely to misfire. The influence of misfire occurrence on the further engine performance was also analyzed.