It is found that the critical temperature in the formation of self-assembled n-hexadecyltrichlorosilane (HTS) monolayers on mica depends on the water content in the solvent dodecane. Under the condition of 35 +/-2 ppm water content in dodecane, the critical temperature is found at 31 +/-1 degreesC. When the water content in dodecane is lowered down to 13 +/-2 ppm, no critical temperature is found in the temperature range of 10-55 degreesC. Our results indicate that the critical temperature is subject to the extent of hydrolysis of HTS molecules in bulk phase of solution. A reaction mechanism of monolayer formation is discussed.