This paper describes the use of surface plasmon resonance (SPR) spectroscopy to measure the rates and extents of association of four detergents-sodium dodecyl sulfate (SDS), beta-octyl glucoside, Triton X-100, and Tween 20-to self-assembled monolayers (SAMs) of alkanethiolates on gold. SAMs presenting hexaethylene glycol groups resisted the adsorption of all four detergents. These same detergents associated with hydrophobic SAMs presenting methyl groups; the concentration of detergent molecules on the surface was 120-280 pmol/cm(2). The associations of the detergents with the hydrophobic SAM were described well by the Langmuir adsorption isotherm. The dissociation constants K-d (M) for the desorption of the detergents from the surface correlated with the critical micelle concentration (cmc, M) of the detergents in solution, and followed the relationship cmc approximate to 7 (+/-2)K-d. The efficacy of SDS in removing the protein fibrinogen adsorbed on a hydrophobic SAM depended strongly on the concentration of detergent. SDS at a concentration three times greater than the cmc removed (or displaced) the adsorbed layer of protein in seconds; SDS at a concentration three times smaller than the cmc did not desorb it even after several minutes. This paper shows that SPR is a useful analytical technique for characterizing the interactions of detergents-and other molecules having low molecular weight-with the well-defined surfaces of SAMs.