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Relative roles of premixed and diffusion burning in diesel combustion have been examined in terms of characteristic times for ignition delay (τig), combustion (τc) and fuel-air mixing (τm). Results indicate that the majority of the diesel combustion process is diffusion controlled, as in gas turbine combustion, since τc << τm over the entire range of operating conditions. During the ignition phase, some premixed burning can occur in the fringe of the fuel spray where τm < τig; however, most of the fuel injected prior to ignition also burns in the diffusion mode, since τm > τig in the majority of the fuel spray. The fraction of premixed burning which occurs during the ignition phase is increased by longer τig, high rates of fuel injection, high air swirl and the use of multi-hole injectors, which increase the surface area of the spray.