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The processes of hydrocarbons diffusing into and out of the oil film are studied. The theoretical simulations show that the magnitude of desorbed hydrocarbons from the oil film increases with the increase of the oil film thickness until a critical one at which it reaches a maximum, and the maximum value changes little when the oil film is thicker than the critical one. Under normal operating conditions, the oil film thickness is approximately equal to or greater than the critical one, so that the oil film nearly has its definite and maximum contribution to the exhausted hydrocarbons. The same conclusion can also be drawn from the experimental data. The measured concentration of unburnt hydrocarbons almost has no difference as far as the different petroleum-based oils and the ring packs are concerned, which may cause the oil film thickness to change. The ‘oil-free’ engine test proves that the oil film may contribute about 28 percent to the total hydrocarbon emissions of SI engine.