Combustion chamber deposits in an internal combustion engine are known to impair engine performance. Using a variable temperature probe and retrievable sampling coupons, this study shows that the deposit-forming rate is inversely related to surface temperature, and directly related to the stabilized deposit weight. Together with the well-recognized fact that deposits are good thermal insulators, a deposit-forming mechanism is proposed. As combustion chamber deposits form, in essence the chamber surface is coated with thermal insulators, layer after layer. Consequently, the surface temperature will rise as the deposit grows.The previously derived critical surface temperature of 310°C is found to be valid in this study for fuel-derived deposits. Furthermore, a critical surface temperature also exists for oil-derived deposits, except that the critical surface temperature is about 60°C higher than that for fuel-derived deposits.