This paper demonstrates that surface plasmon resonance (SPR) spectroscopy can be used to measure the nonspecific adsorption of proteins to self-assembled monolayers (SAMs) of alkanethiolates on gold in situ and in realtime. Mixed SAMs comprising hexa(ethylene glycol) and methyl groups that have values of (chi Me) < 0.5 resisted the adsorption of four test proteins : RNase A, lysozyme, fibrinogen, and pyruvate kinase. These four proteins adsorbed irreversibly to surfaces having values of (chi Me) > 0.5 : the amount of adsorbed protein correlated with (chi Me). The initial rate for adsorption of fibrinogen to a methyl-terminated SAM ((chi Me) = 1.0) followed first-order kinetics. The combination of SAMs and SPR described here is particularly well suited for investigations of the interactions of proteins with structurally well-defined organic surfaces.