Low-density self-assembled monolayers (SAMs) on gold were generated by the adsorption of a series of specifically designed 2-monoalkylpropane-1,3-dithiol derivatives, CH3(CH2)(n)CH[CH2SH](2), where n = 11, 13, 14. The monolayers were characterized by optical ellipsometry, X-ray photoelectron spectroscopy, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy, and sum-frequency generation. Comparison of these data to those collected on SAMs generated from normal alkanethiols, CH3(CH2)(n+2)SH, and 2,2-dialkylpropane-1,3-dithiol derivatives, [CH3(CH2)(n)](2)C[CH2SH](2), of similar chain length suggests that the new "monoalkanedithiol" SAMs are the least crystalline, exposing both methyl and methylene groups at the interface due to the low density of alkyl chains. Further comparison of these low-density SAMs to those obtained on branched and linear polyethylene films suggests that the exposure of interfacial methylene groups is greater for the polymer films.