To reveal the spray processes in a diesel engine, with special regards to the gasification and thermal decomposition of the injected fuel at an elevated air temperature, a study has been carried out on a simulated two-cycle engine using a fast gas-sampling technique. The results obtained suggest that already in the period before an appreciable pressure rise in the cylinder is observed, in other words, even during the so-called ignition delay period, the full injected quickly gasifies and decomposes, forming a lot of split hydrocarbons. This phenomenon may be partly attributed to the fact that a hot burnt gas formed in the vicinity of the spray jet is sucked into the spray cone, thus promoting the gasification and thermal decomposition. It is suggested that an amount of decomposed products which undergoe turbulent mixing will accumulate in the doanstream of the fuel jet.