Abstract: A method for determining both connate water saturation and salinity in an oil-bearing reservoir with a single invasive well test is disclosed. In this method, a fluid, miscible with the reservoir oil and having the ability to dissolve a limited amount of the connate water where the solubility of the water in the fluid depends on the salinity of the water, is injected into a well penetrating the reservoir. The reservoir is then produced until oil breakthrough occurs. Samples of the produced fluids are analyzed for their water content. This water content is plotted as a function of the volume of fluids produced. The resulting curve is compared to similar curves generated by mathematical simulation.