Superoxide ion-radical, O-2, the one-electron reduction product of molecular oxygen, is a long-suspected first intermediate in chemical reactions using oxygen as the ultimate electron acceptor. We have detected photochemical production of O-2 in a variety of eastern Caribbean waters studied in Spring and Fall by using a chemically direct O-2 trapping reaction combined with 15N and/or 18O isotopic labelling of reactants to provide sensitivity and specificity. Photochemical superoxide production rates ranged from 0.1--6 nmol L-1 min-1 of full-sun irradiation in Spring and from 0.2--8 nmol L-1 min-1 in Fall. These superoxide fluxes correlate well with independently measured total (NO-scavengeable) radical fluxes in both seasons and account for ~1/3 of the total radical production. Hence we confirm that O-2 is a quantitatively important, perhaps dominant, reactive transient species in the photochemical radical array. Semiquantitative estimates suggest that O-2 dismutation to HOOH and O2 can account for most or all the photochemical HOOH production in surface waters. O-2 production rates also correlated negatively with salinity. ¿ American Geophysical Union 1993