A reasonably good correlation has been found between overlying water salinities and the pyrite-to-"FeS" (acid-volatile iron monosulfides) ratio in modern anoxic sediments below the zone of sulfate reduction. Values of FeS 2 /FeS greater than 10.0 are characteristic of marine sediments, whereas ratios from brackish estuarine or fresh water lake sediments are generally less than one. The low ratios are believed to result from an incomplete conversion of FeS to pyrite. Since the reaction is dependent upon available interstitial sulfate, it cannot proceed to completion in low sulfate-containing fresh or brackish water sediments. It is suggested that the presence of appreciable authigenic iron monosulfides in ancient sediments may be used as evidence that they were deposited in brackish or fresh water environments.