A single-cylinder engine was used to study the effect of engine operating parameters on the early stage of deposit formation (first 8 hours). Deposit samples were collected from the engine cylinder using removable sampling probes.Among the engine operating parameters studied, coolant temperature had the greatest influence on deposit formation. Equivalence ratio of the air-fuel mixture was also important. Other variables such as compression ratio and intake air temperature had minimal effects.Investigations using a temperature controlled probe revealed that surface temperature is a dominant factor in the deposit forming process. Within a temperature range from 98°C to 256°C, there is an inverse relationship between the amount of deposit accumulated and the surface temperature. Extrapolating the experimental data showed that the critical surface temperature for deposit formation is near 310°C, above which no deposit is expected to form.