The structures and formation mechanisms of self-assembled monolayers of n-alkyltrichlorosilanes on glass substrates have been examined systematically using Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). The adsorption temperature dependency of FTIR frequencies and intensities of CH2 stretching vibration bands indicates the existence of a characteristic temperature, T-c, for each monolayer. The hydrocarbon chains observe an ordered and closely packed state when the monolayers are prepared below T-c, whilst disordered monolayers with low molecular density are formed above T-c. AFM observation clearly demonstrates an island-type growth of monolayers prepared below T-c. The results obtained contribute to an understanding of the growth behavior of the monolayers and the physical meaning of the T-c.