The rate of the reaction between solution-phase myoglobin and SAMs fabricated from omega-X-hexadecylthiomethylbenzenes where X was a mesylate, chloride, or bromide was investigated using surface plasmon resonance (spr). The data were fit to a model in which nonspecific adsorption competes with a second-order substitution reaction between myoglobin and the surface species, The second-order reaction rate constants obtained from the regression analysis were 440 +/- 140 M(-1)s(-1) for the bromide, 56 +/- 10 M(-1)s(-1) for the mesylate, and 38 +/- 12 M(-1)s(-1) for the chloride. The experimental rate law fit to the data and the increase in rate with decreased basicity of the leaving group were consistent with an S(N)2 mechanism.