Functionalized alkanethiols (-CH3, -NH2, -OH, -COOH, -SH, -SO3H, -CN) of different lengths, self-assembled on Au(111), were studied with cyclic voltammetry and in situ scanning tunneling microscopy. The surface structure was determined under potential conditions at which only a dielectric behavior is expected. In general, the size of the terminal group together with the chemical interaction between neighboring thiol molecules plays a crucial role for the resulting adlayer structure. Shorter-chain thiols with bulky endgroups frequently lead to lower density coverages and more distorted packings.