Sodium dodecyl sulfate (SDS), adsorbed from aqueous solution onto a monolayer of dimethyloctadecylsiloxane (C18) bonded to silica, was characterized spectroscopically. The structural and dynamical properties of the adsorbed SDS were probed by the fluorescence anisotropy of a dilute hydrophobic fluorophor, 1,4-bis(o-methylstyryl)benzene. Just below the critical micelle concentration (cmc) of 8.3 mM, the orientational distribution of the probe is narrowed and tilted more toward the surface normal, and its rotational diffusion slows, as compared to its behavior above the cmc. These properties below the cmc were most extreme at 7 mM in SDS concentration. FTIR spectroscopy was used to measure the amount of adsorbed SDS, and it revealed that there is about 3 times as much SDS adsorbed below the cmc as there is above it. A distinct maximum in the adsorption isotherm occurs at 7 mM, the same concentration at which the rotational diffusion behavior is most extreme. The FTIR spectral bands are narrowed and shifted below the cmc, showing that the adsorbed SDS in the premicellar region is organized densely.