Isolation of native petroleum surfactants for lowering interfacial tensions in aqueous-alkaline systems. Petroleum of the type having reduced interfacial tension with water at high pH is subjected to fractionation by distillation including distillation under vacuum in the temperature range of 100.degree.-200.degree. C. to recover a surfactant fraction of the petroleum distilled in that temperature range. The fraction can be further concentrated by removing benzene-soluble components to obtain an ether-soluble residue having a density greater than water. The fraction alone, mixed with petroleum, or enriched with the residue, or the residue mixed with petroleum, can be injected as a slug into a subterranean oil reservoir to enhance tertiary recovery by alkaline water flooding. The amenability of a reservoir for alkaline flooding can be determined by separating the native surfactant fraction and measuring its interfacial tension with water at the proposed alkalinity of the floodwater.