A model has been developed which predicts engine-out hydrocarbon (HC) emissions for spark-ignition engines. The model consists of a set of scaling laws that describe the individual processes that contribute to HC emissions. The model inputs are the critical engine design and operating variables. This set of individual process scaling relations was then calibrated using production spark-ignition engine data at a fixed light-load operating point. The data base consisted of engine-out HC emissions from two-valve and four-valve engine designs with variations in spark timing, valve timing, coolant temperature, crevice volume, and EGR, for five different engines. The model was calibrated separately for the three different engines to accommodate differences in engine design details and to determine the relative magnitudes of each of the major sources. A good fit to this database was obtained.The model provides insight into the individual phenomena involved in the total HC mechanism and how each individual process is affected by model inputs. The model results showed that the fuel sources (oil layers, deposits, and liquid fuel) and fuel-air sources (crevices and quench layers) contributed approximately equally to the engine-out HC emissions. Typical in-cylinder post combustion HC oxidation levels were 68% for fuel-air sources and close to zero for fuel sources. For all sources, about 25% of the unoxidized HC were retained in the cylinder, and exhaust port HC oxidation was about 35%. The model predicted that the fuel source contribution to the engine-out HC emissions was most affected by changes in coolant temperature, compression ratio, EGR, and spark timing, whereas the contribution from the fuel-air sources was affected most by crevice volume, spark timing and compression ratio, and only slightly by EGR, valve overlap and coolant temperature. The model was used to establish relative sensitivities of HC emissions to critical design and operating parameters by determining the magnitude of individual parameter changes that produced a 10% decrease in HC emissions.