The process of selecting an effective surfactant for wettability alteration is dependent on a number of factors, including mineral type, temperature, salinity, and nature of adsorbed oil and ultimately how the molecular structure of the surfactant interacts with all of these. Here, we present an experimental study of the effectiveness of nonionic surfactants with different hydrophobic groups and different lengths of hydrophilic ethylene oxide oligomers. The surfactants selected alter the wettability of the rock primarily by acting on the water-rock and oil-rock interfaces. The dynamics of wettability alteration is measured by the evolution of contact angles of oil drops on initially oil-wet surfaces at three different temperatures. Wettability alteration is found to be enhanced by surfactants with shorter hydrophilic units and increased temperatures. Experimental observations are efficiently summarized by a few thermodynamic and kinetic parameters. Qualitative experiments are performed to study surfactant-induced dewetting of oil films. Finally, a model involving "coating" and "sweeping" mechanisms is proposed to explain the mechanism of surfactant action.