The chemical composition of fluid inclusions trapped within the marine-dominated Permian evaporite sequence of the Palo Duro Basin, Texas, departs significantly from that expected in the evolution of seawater through evaporative concentration and mineral precipitation. The origin of these fluids is evaluated in the context of the sedimentary and diagenetic environments in which the halites evolved. The petrography and fabric show that many of the halites escaped advanced recrystallization, suggesting that the fluids entrapped in such samples are original brines. The seawater-derived brine-pool and diagenetic brines from which the halites precipitated were modified as a result of fluctuations in water level on an evaporite flat and of a complex hydrological system through which seawater was fed into the basin and refluxed. The ionic ratios between Ca, Mg and SO 4 are influenced by dolomitization and associated reactions of interstitial brines with carbonates adjacent to the halite beds. Formation and subsequent dissolution of efflorescent evaporite crusts that are not in equilibrium with subaqueously precipitated evaporites shift the ratios of Mg, Na, K, and Cl to Br away from the expected path of evaporative seawater concentration. Syndepositional alteration of clay and feldspar minerals probably also occurred, but these effects cannot be evaluated from the present data. The variability and complexity of brine evolution within an uncomplicated evaporite setting, such as that of the Palo Duro Basin, has important implications for interpretation of the origin and history of evaporite-influenced brines in other basins. Laboratory evaporation experiments and analyses of salt pan brines only serve as a basic reference for the evaporation path of seawater; they ignore the early diagenetic processes, and hence taken alone are inadequate analogs for ancient marine evaporite environments.