Adsorption of trimethylamine (TMA) on Si(100)c(4x2) has been studied using scanning tunneling microscopy (STM), valence photoelectron spectroscopy (PES), and high-resolution electron energy spectroscopy (HREELS) between 80 and 300 K. The TMA molecule is adsorbed nondissociatively only on the down dimer atom and appears as a triangle shaped bright protrusion in the occupied state STM image. The saturation coverage of TMA on Si(100) is estimated to be similar to0.25 ML (ML = one molecule per surface Si atom). The PES and HREELS results suggest that TMA is adsorbed through the N lone pair by forming an Si-N dative bond. The HREELS spectra indicate that the symmetry of adsorbed TMA is C, and the molecular C-3 axis is tilted from the surface normal. The stretching mode of the SiN dative bond is observed at 525 cm(-1). Thus the combined STM, PES, and HREELS results indicate that the chemisorption of TMA on Si(100)c(4x2) occurs selectively on the down dimer atom via dative bonding. Hence, the TMA adsorption on Si(100)c(4x2) is purely site-specific on the down dimer atom and can be categorized in Lewis acid-base reaction. The acidity of the down dimer atom is also discussed according to the analysis of vibrational spectra.