In situ dissolved carbon monoxide (CO) in oligotrophic waters follows a diel cycle varying from 0.3 to 0.5 nmol L-1 before dawn to 2.5 to 3 nmol L-1 in early afternoon, when photo-production of CO exceeds biological CO oxidation and other sinks. Coastal waters may contain up to 15 nmol L-1 [CO] in the daytime. Assays to measure the rate of CO bio-oxidation typically involve the addition of labeled CO to sealed samples, resulting in CO concentrations that are above ambient levels during incubation (up to 9 nmol L-1 CO). We find that biological oxidation of CO obeys first-order kinetics when incubated with up to 4 nmol L-1 [CO] in coastal water samples and up to between 4 and 10.8 nmol L-1 in oligotrophic waters. At higher [CO], kinetic behavior transitions to zero-order or saturation kinetics. CO–oxidation rate coefficients obtained in dark incubations were not representative of the entire diurnal period, as others have assumed. Biological CO–oxidation rate coefficients kco measured in dark incubations of Sargasso Sea surface water in summer were 0.020 ± 0.002 h-1 (mean ± standard deviation) and an order of magnitude greater than those measured in situ during daylight hours (0.002 ± 0.001 h-1). Dark and in situ rate coefficients in early spring were 0.006 ± 0.004 h-1 and 0.003 ± 0.001 h-1, respectively. In dark incubations of Vineyard Sound water, kco was 0.127 ± 0.038 h-1. The apparent half-saturation constant Kapp for CO ranged from 2.04 to 5.44 nmol L-1 CO in both environments.