Poly(2-hydroxyethyl methacrylate) (PHEMA) films were grown onto gold via water-accelerated, surfaceinitiated atom transfer radical polymerization (ATRP), and the resulting side chains were modified by reaction with alkanoyl chlorides (C m H 2m+1 COCl; m ) 1, 7, 11, 13, 15, and 17) to incorporate hydrocarbon side groups within the film. We have previously demonstrated the ability to react ∼70-80% of PHEMA hydroxyl side chains with fluorocarbon acid chlorides to prepare partially fluorinated films. Here we convert the side chains to hydrocarbon esters with diminishing conversion (80 to 40%) as m is increased and compare the resulting films with fluorocarbon-modified PHEMA. Based on IR spectra and wetting data, hydrocarbon side chains structure the film to a greater extent as m is increased. A critical chain length (m ) 15) was required to orient chains normal to the interface at the outer film surface and impart the wetting properties of a dense methyl surface. The resistances of the films against the transport of redox probes were greatly enhanced with increasing conversion of hydroxyl groups within the film and were modestly affected by film structuring to create a densely packed methyl surface. For example, capping the hydroxyl groups of PHEMA by reaction with acetyl chloride resulted in an unstructured film with >90% conversion that increased film resistance by almost four orders of magnitude over the base PHEMA film without a significant volumetric enhancement of the side chains. Also, the decreasing conversion as m is increased from 7 to 17 resulted in a decreasing film resistance even though the longer chains structured the film and surface to a greater extent. These results illustrate the important effect on barrier properties of unreacted hydroxyl groups, which possibly associate to create water and ion-diffusing pathways within the film. The barrier properties of these hydrocarbon-modified PHEMA films are comparable to those of fluorocarbon-modified PHEMA when both conversion and the molecular weight of the modifying group are similar.