Hydrophobically modified sodium humate surfactants with different chain lengths (C-n-HANa, n = 10, 12, 14, 18) were synthesized in this paper and the molecular structures were confirmed by FT-IR and UV-vis spectra. The effects of C-n-HANa, alcohol (ethanol, n-propanol, iso-propanol, n-butbanol) and alkaline (KOH, NaOH, Na2CO3, Na3PO4 center dot 12H(2)O, NaOH: Na2CO3) on the interfacial tension of oil/water interface were investigated by KRUSS SITE-100 spinning drop interfacial tension apparatus. Ultra-low interfacial tension related to enhanced oil recovery (EOR) was obtained at the crude oil/water interface from the mixture solution of the C-n-HANa surfactant, Na3PO4 center dot 12H(2)O and n-propanol. The experimental results show that hydrophobic modification of sodium humate surfactants cause low interfacial tension at the crude oil/water interface as compared with those of the single sodium humate. While the interfacial tension of the C-n-HANa/crude oil interface could reach to 10(-1) mN/m, the interfacial tension of the C-n-HANa/Na3PO4 center dot 12H(2)O/crude oil system, in aqueous solution, decreases so much that sometimes it is less than 10(-4) mN/m. Furthermore, n-propanol is added to the C-10-HANa/Na3PO4 center dot 12H(2)O/crude oil system, it can make the range of the ultra-low interfacial tension extend from 0.20-0.30% to 0.20-0.70% for Na3PO4 center dot 12H(2)O, and the minimum interfacial tension is 5.27 x 10(-5) mN/m at Na3PO4 center dot 12H(2)O concentration about 0.35%. (C) 2012 Elsevier B. V. All rights reserved.