This Letter reports that n-alkanethiolate self-assembled monolayers (SAMs) formed onto gold from aqueous micellar solutions of hexaethylene glycol monododecyl ether (C12E6) exhibit greater overall chain densities and are more electrochemically blocking than those formed from traditional organic solvents such as ethanol. As determined by reflectance infrared spectroscopy, the alkyl chains in SAMs formed from 10 mM C12E6(aq) are less canted on average than those in SAMs formed from ethanol. This greater overall chain density for the water-borne SAMs is attributed to hydrophobic interactions between alkyl chains during assembly. Consistent with a more defect-free structure, the water-borne SAMs exhibit greater resistances against the diffusion of redox probes than SAMs formed in ethanol.