The adsorption of premicellar sodium dodecyl sulfate (SDS) onto methylated (C-1) silica surfaces was investigated spectroscopically to explore the physical origin of premicellar adsorption. Methylation was done to provide a hydrophobic surface without interpenetration of the SDS chains into the monolayer. The premicellar adsorption maxima were found to be smaller for the C-1 surface than for the n-octadecylsiloxane (C-18) surface, which implicates interpenetration as an important factor in premicellar adsorption maxima. Roughness was found to increase the premicellar adsorption maximum on the C-1 surface. The rotational diffusion behavior of acridine orange on the C-1 and C-18 surfaces paints to a structural transition in the adsorbed SDS layer as the origin of the large premicellar adsorption maximum of SDS on C-18.