An investigation of the genetic environment of laminar and massive gypsum and intercalated carbonates (Messinian, central Sicily) has been carried out by means of isotopic, textural, and mineralogical analyses. The oxygen and hydrogen isotope compositions of crystal water from the laminar gypsum indicate primary precipitation of the sulfate from marine waters concentrated by evaporation. The isotopic data also show compositional changes due to periodic influx of continental waters into the deposition basin. In the laminar gypsum and interlayered dolostones we find a micritic, near-to or stoichiometric dolomite showing very positive delta 18 O and negative delta 13 C values which are consistent with formation of this carbonate, at a very early diagenetic stage, by precipitation from highly evaporated interstitial waters depleted in SO 4 (super --) ions by gypsum crystallization and concurrent bacterial SO 4 (super --) reduction. Open-marine-water incursions probably led to the deposition of carbonate layers overlying the laminar gypsum. These contain abundant aragonite whose textural features and isotopic composition suggest precipitation in shallow and quiet marine waters slightly enriched in 18 O by evaporation. The isotopic data of a euhedral and Ca-rich dolomite, formed later than the aragonite, may reflect further influxes of continental waters into the basin. On the basis of delta 18 O and delta D values, the massive gypsum in the upper portion of the sedimentary sequence seems to have been formed by hydration of anhydrite.