The influence of the inter-ring crevice, the volume between the top and the second piston rings, on unburned hydrocarbon (UHC) emissions has been numerically investigated with the aid of experimental results. The main goal of this study was to estimate the level of UHC emissions induced by the blow-up of the unburned gas trapped in the inter-ring crevice (inter-ring mixture). For this purpose, the relationship between the inter-ring mixture and UHC emissions was established from the experimental results. A physical flow model integrated with a ring dynamics model was constructed to predict the gas flows through the inter-ring crevice. Calculated results showed that some of the inter-ring mixture returned to the combustion chamber, while the cylinder pressure fell below the inter-ring pressure late in the expansion stroke. The amount of inter-ring mixture returning to the combustion chamber after exhaust valve open was calculated and converted to the corresponding UHC emissions using the relationship between the inter-ring mixture and UHC emissions obtained from the experiments. The calculated level of UHC emissions caused by the inter-ring mixtures was 10-30 per cent of the entire UHC emissions over a range of speeds (1250-3500 r/min) and loads [185-556 kPa brake mean effective pressure (b.m.e.p.)]. The contribution was highest at the medium speed and medium load (2500 r/min and 432 kPa b.m.e.p.) for the test condition, which corresponds to the engine condition frequently required for normal running. These findings confirm the importance of inter-ring crevice volume in the UHC emissions in spark ignition engines.