The effect of surface-active organic matter generated by seven common species of marine phytoplankton on gas exchange rates under turbulent conditions at the air-water interface was determined. Reduction in oxygen evasion rates ranging from 5 to 50% were observed relative to clean seawater controls. Relative oxygen exchange coefficients (expressed as R=Kw/Kw) were shown to be sensitive to small changes in total dissolved carbohydrate at concentrations <1 mg C (carbon) L-1 and to asymptotically decrease to a lower limit (R=55--70%) at concentrations between 2 and 6 mg C L-1. A corresponding relationship was observed in which R decreased with increasing relative surfactant amounts derived from surface pressure-area measurements. However, gas exchange reductions were significant for plankton exudate samples displaying surface pressures ≲1 mN m-1. It thus seems that condensed monolayer films are not a prerequisite for reduced gas exchange and that relatively soluble surfactants derived from phytoplankton can strongly affect the dissipation of near-surface turbulence and lead to changes in the Schmidt number dependency of Kw. Based on detailed analyses of carbohydrate-containing surface-active exudates isolated by solid phase extraction from one of the species, Phaeodactylum tricornutum, it appears that small glucans and heteropolysaccharides associated with proteins and possibly lipids were responsible for the observed reductions in R. ¿ American Geophysical Union 1990