An experimental study has been con-ducted to investigate the effects on water flood oil recovery caused by the addition of sodium hydroxide to only a portion of the injected fluid. The results of inter-facial tension measurements between the viscous crude oil and water indicate that a low interfacial tension can be achieved at concentrations of sodium hydroxide varying from 0.1 to 7.0% by weight. The wide range of concentrations is a result of the signif-icant variation in water compositions.The results of displacement tests in-volving an artificial sodium chloride brine indicate that a significant increase in recovery can be achieved at concentrations of sodium hydroxide in excess of 0.1% by weight. The results of displacement studies involving varying slug sizes indic-ated that sodium hydroxide concentrations as low as 0.1% by weight in a slug volume of approximately 40% of the pore volume, resulted in an oil recovery equivalent to that which could be obtained by the contin-uous injection of a dilute aqueous sodium hydroxide solution of 0.1% or greater by weight in caustic concentration.The optimum concentration of sodium hydroxide required to significantly reduce the interfacial tension and thus increase the recovery of viscous crud 76-05