Photokinetic studies demonstrate that sulfide is consumed in seawater upon light exposure. The half-life (t1/2) of sulfide (10 μM) added to Biscayne Bay water and Gulf Stream water are 49(¿15) and 147(¿10) min, respectively. Both UV and visible radiation were shown to accelerate the photodecomposition of sulfide. However, due to a higher amount of radiation in the visible range reaching the sea surface, we predict that this portion of the light spectrum will be most important in inducing photoreaction of sufide in seawater. This as yet unaccounted sink of sulfide in seawater could be responsible for the sulfide daytime low and nighttime high concentration values observed by several investigators. This finding further unbalances the sulfide budget and reinforces the need for systematic research on the role of photochemical processes on sulfur species in seawater.¿ 1997 American Geophysical Union