A test of the hypothesis that hydrogen-rich organic matter accumulates preferentially in sediments under oxygen minima has been carried out by examining the composition of laminated and homogeneous diatomaceous sediments of the Guaymas Basin in the Gulf of California. The laminated sediments occur where an intense oxygen minimum intersects the slope around the margins of the basin. Above and below the depth of the oxygen minimum, the sediments are homogenized by a burrowing infauna. The two sediment types have very similar bulk chemical and mineral compositions, and the organic matter is overwhelmingly marine. The total organic carbon contents and Rock-Eval parameters (hydrogen and oxygen indices) of the two sediment types are not significantly different. Whereas a road organic carbon maximum in the homogeneous sediments is centered at roughly 1200 m depth on the slope of the basin, this maximum lies deeper than the core of the oxygen minimum. Thus, both the concentration and the nature of the organic matter in the sediments of this highly productive marginal marine basin are not related to the concentration of dissolved oxygen in the bottom waters.