The purposes of this series of studies are to examine the characteristics of intake-valve deposits and to clarify their formation mechanism. This paper discusses the chemical compositions and physical states of the deposits on the basis of the results obtained by chemical analyses.The results indicate that the deposits mainly originate from engine oil, and suggest that the main reaction route of the deposit formation is the carbonization of oxidized engine oil. Namely, low boiling point (light) fractions in the engine oil easily evaporate from the valve surface. On the other hand, the remaining heavy fractions are subjected to deposit forming reactions. These final states are amorphous and carbonaceous aggregates.The deposit has a tendency to form in the temperature range of about 230 to 350 °C. The location of accumulating deposit depends on the valve surface temperature.The fuel injection onto the intake-valve reduces the accumulation of the deposit and surpresses carbonization reactions, due to the dissolving and cooling actions of fuel.