The low-energy reactive collisions of two independent probe ions, pyrazine and d(6)-benzene, illustrate an odd-even hydrocarbon chain length effect for a wide range of hydrocarbon self-assembled monolayer (SAM) surfaces. SAM surfaces prepared from alkanethiols ranging from CH3(CH2)(10)SH to CH3(CH2)(17)SH chemisorbed to polycrystalline gold are shown to exhibit an odd-even effect where the orientation of the terminal methyl group determines the reaction behavior of the thin film. X-ray photoelectron spectronscopy shows the surfaces to be homogeneously covered and provides evidence for the presence of a thiolate (Au-SR) on the SAM surface. When 30 eV incident ions were used, the extent of hydrogen addition to the incident probe ion was larger for odd carbon chain length SAM surfaces when compared to the even chain length films. For an odd chain length SAM surface, the terminal methyl group exposes a C-H bond perpendicular to the surface, increasing hydrogen addition reactivity for the probe ions. The low-energy probe ions also reacted with the surfaces to form alkyl addition products. The alkyl addition products also showed an odd-even effect. In this case, the even chain length SAM surfaces were more reactive than the odd chain length surfaces. For an even SAM surface, the terminal C-C bond is oriented quasi-perpendicular to the Au surface, allowing more direct access to the carbon atom on the terminal methyl group and increasing its reactivity.