An isotopic study of sulfide minerals associated with sandstone-type uranium deposits of the Colorado Plateau and Wyoming indicates S 32 /S 34 ratios of these 57 samples varying between 21.93 to 23.32. The majority, however, exhibit ratios that are highly enriched in the lighter isotope compared to the ratio of primordial sulfur that is assumed to be similar in ratio to meteoritic troilites, which vary in ratio from 22.18 to 22.24.The samples are predominantly pyrite and marcasite, but chalcopyrite, covellite, and bornite are included, even though the sulfide mineral species appears to have no relationship to the specific ratio obtained. There is, furthermore, no indication of a difference between ratios obtained from sulfides collected within uranium ore zones in comparison to ratios of sulfides occurring in either adjacent or distant barren uranium ground. The formations (Triassic to Eocene) from which the samples were collected have no obvious correlation with the ratios, nor does the presence or absence of lignitic, woody, or other carbonaceous matter show any correlation with the ratios.The relatively broad spread of these high ratio values, especially of those samples collected from one deposit, is very suggestive of hydrogen sulfide derived from anerobic bacteria. It is suggested that sulfate waters (possibly both connate and meteoric in origin) in Mesozoic sediments of the Colorado Plateau area were reduced by anerobic bacteria to hydrogen sulfide when these waters encountered carbonaceous-rich zones that provided an environment and energy source for the bacteria. The hydrogen sulfide may have remained where formed or may have migrated to locales barren of carbonaceous matter, but most likely did both.At a later date, about 60 to 75 million years ago, ore solutions moving through these more porous and permeable channels encountered the very effective reducing agent of hydrogen sulfide which brought about the concentration of uranium through precipitation of the soluble uranyl ions to relatively insoluble UO 2 . At the same time iron (and in some cases copper) sulfate was precipitated as relatively insoluble sulfides.