The Viséan evaporites of the Windsor Group in the Moncton Subbasin at the western limit of the Maritimes Basin of eastern Canada represent an important stratigraphic sequence to study in determining changes in the chemistry of marine brines. Seventeen samples of halite have been selected for fluid inclusion study. The chemical composition of inclusion brines was examined using the Petrichenko method of glass capillaries with applied ultramicroanalytical techniques. The results of the study indicate that the conditions of evaporite deposition in the Windsor Basin differed from those recorded in other Phanerozoic basins and that the history of the Windsor Basin brines was characterized by changes in the chemical composition of the brines. It was also found that there existed continual unfavourable conditions for the day-and-night zonality in chevron halite crystals and for the origin of relatively large (>50 μm) fluid inclusions and that the gas content was low not only during the deposition but also during the diagenesis of potash deposits. The chemical composition of the Viséan waters of the Moncton Subbasin at the beginning and end of deposition of the Windsor Group evaporite sequence was controlled by the predominating waters of nonmarine origin; during the potash deposition the controlling factor was chloride-type marine waters. In general, the geochemical data support the concept of Carboniferous evaporite basins with Na–K–Mg–Ca–Cl-type brines.