Experimental investigations have been conducted in order to elucidate the mechanism responsible for wettability alteration of quartz surface in the presence of nonionic surfactants. The wettability alteration has been verified by measuring the contact angle on a quartz-crude oil-distilled water system by systematic variation of surfactants, temperature, and water salinity. In all cases, contact angle decreases with elapse of time. As ethylene oxide number of nonionic surfactant increases the initial equilibrium contact angle decreases. Results also show that as temperature increases, contact angle decreases due to reduction of oil viscosity. With increasing water salinity, contact angle decreases up to a certain salinity and then increases. This salinity may be termed as optimal salinity for the system. Scanning electron microscopy analysis shows the roughness of the quartz surface. Infrared spectroscopy analysis of the quartz sample indicates the presence of Si-O groups. Fourier transform infrared (FTIR) spectroscopy and acid number analysis of crude oil suggest the acidic nature of the oil. Critical micelle concentrations of the surfactants were also measured by the surface tension method. The lowest surface tension value achieved by Tergitol 15-S-12 is 28 mN m(-1).