A comparative study of the growth processes of self-assembled monolayers (SAMs) of thiophene and alkylthiophene on Au(111) was made by Fourier-transform infrared reflection absorption spectroscopy (FTIR-RAS). Measurements of temporal peak intensity and the time-dependent Langmuir adsorption isotherm revealed two phases with different molecular orientations during SAM growth of thiophene. In the primary stage, thiophene in the SAM orients parallel to the gold surface. In the second stage, the molecule reorients to a standing-up configuration with respect to the surface. Introduction of alkyl chain to thiophene ring does not essentially change the growth process, but induces a faster growth rate than that of the thiophene SAM. The intermolecular forces of adsorbate, e.g., van der Waals force, play a role as a rate-limiting factor for the self-assembly of adsorbates on the solid surface.