Scanning electrochemical microscopy (SECM) was employed to study the rate of the second-order homogeneous reaction of a species electrogenerated at an ultramicroelectrode tip. Oxidation of 4-nitrophenolate in acetonitrile to the phenoxyl radical took place at a gold ultramicroelectrode (5-mu m diameter) held in close proximity to a larger substrate electrode that detected the tip-generated species. A radical dimerization rate constant of 1.2 (+/-0.3) X 10(8) M(-1) s(-1) was determined. Approximate analytical equations for the treatment of irreversible kinetics following electron transfer were developed to simplify the analysis of feedback and collection data for both first- and second-order reactions. The effect of possible interference by the reference material added to allow precise determination of the tip-substrate spacing (benzoquinone or ferrocene) was also investigated.