maxima of these species should have been found near the oxic/anoxic interface, which is around 80-m depth. The relatively low concentrations of sulfite and thiosulfate near the surface suggest that other factors, such as bacterial activities, may be important in controlling their distribution, and that both sulfite and thiosulfate are turned over rapidly in these waters.The reasons for the gradual increase in concentrations of sulfite and thiosulfate with depth (down to ~2000 m) are not clear. It is possible that some sulfite could have formed by air oxidation of sulfide during sampling, since there was a brief exposure to air (<30 s) just prior to derivatization. However, from kinetic considerations, this brief exposure was not likely to have given rise to the observed high concentrations in the deep samples ( 14). Furthermore, the sulfite depth profile closely parallels that of thiosulfate, which has a much slower formation rate than that of sulfite during air oxidation of sulfide in Black Sea water ( 14). An earlier work reported more than 20 µ concentrations of thiosulfate in the Black Sea waters deeper than ~300 m and attributed bacterial processes for its formation ( 15). The similarity of sulfite and thiosulfate profiles to that of sulfide suggests a common origin of these species in bacterial sulfate reduction. In fact, studies by Millet using radiolabeled sulfate revealed that sulfite was formed as an intermediate in the conversion of sulfate to sulfide (16). Since sulfite as well as thiosulfate are also formed as intermediates in the oxidation of sulfide to sulfate (17,18), it is likely that the reverse reactions occur during sulfate reduction. Our results imply that both sulfite and thiosulfate are probably intermediates in bacterial sulfate reduction.