A distinct odd even effect on the electrical properties, induced by monolayers of alkyl-phenyl molecules directly bound to Si(111), is reported. Monomers of H2C=CH- Even (CH2) phenyl, with n = 2-5, were adsorbed onto Si I-I and formed high-quality monolayers with a binding density of 50-60% Si(111) surface atoms. Molecular dynamics simulations suggest that the binding proximity is close enough to allow efficient /r,r interactions and therefore distinctly different packing and ring orientations for monomers with odd or even numbers of rnethylenes in their alkyl spacers. The odd even alternation in molecular tilt was experimentally confirmed by con. tact ellipsometry, and XPS with a close quantitative matc,t to the simulation results. The orientation. an even s of bot h ring ng p ane and the long axis f the alkyl spacer are more perpendicular pi-pi the with substrate lane for molecules number of niethylenes than for those with an odd number ofi7rriethilYlenesd. with Interestingly! those th an even number conduct better than the effectively thinner mono.laYers Of the Plialeatrin_s le with respect odd number of inethylenes. We attribute this to a change in the orientation of the electron density,on the aromatic to the shortest tunneling path, which increases the harrier for electron transport through the Odd monolaye,rsof,The hig h sensitivity of molecular charge transport o orientation t the oentation of an aromatic moiety might be relevant to better control over th electronic properties of interfaces n organic electronics.