We studied the self-assembly of n-alkanethiols CH3(CH2)(m)SH (m = 5, 7, 9, 11, 13, 15, 17, 19, 2 1) onto platinum thin films with two different surface properties: metallic and oxide. Water contact angle, ellipsometry, and reflection-absorption infrared spectroscopy (RAIRS) measurements suggested that well-organized, methyl-terminated monolayer films formed on the metallic platinum surface. On the other hand, the molecular films formed with the alkanethiol molecules on platinum oxide surfaces were loosely packed and poorly ordered. For the monolayer films formed with alkanethiols of different chain lengths on metallic Pt surfaces, a transition from slightly disordered to crystalline-like structure was observed with increasing chain length. Based on the intensities of the methylene and methyl C-H vibration modes in the RAIRS, the tilt angle of the alkyl chain in the monolayers on Pt surfaces was estimated to be less than 15degrees with respect to the surface normal. X-ray photoelectron spectroscopy revealed the formation of metal-thiolate bonding of the thiol headgroups in the alkanethiol molecules with Pt atoms on the surfaces.