Injection of plain or modified water is one of the methods used to enhance the recovery of oil. A prior knowledge of the effects of injection fluids on the reservoir rock minerals and interstitial fluids is essential for designing a proper enhanced recovery program. A Scanning Electron Microscope (SEM) can be used to evaluate qualitatively the physicochemical changes that are taking place during waterflooding. Studies have been carried out by the writers on cores from an oil field situated in Gujarat State, India. This sandstone reservoir has medium-viscosity crude oil, with favorable acid number, low concentration of Ca⊃++ ⊃ and Mg⊃++ ⊃ cations in formation water, small quantity of water-sensitive clays, and relatively poorly cemented fines. Plain water (economy considerations), polymer solutions (for mobility control), and dilute caustic solutions (for improving displacement efficiency and for mobility control) were used as injection fluids in a large number of laboratory experiments in order to select the optimum parameters, such as concentration and slug size.