Two series of double-chain single-head nonionic surfactants, Guerbet alcohol ethoxylates (C(m)GA-E-n; m = 16 and 20, n = 5.7-24.0), were synthesized, and their performances as surfactants for surfactant-polymer (SP) flooding free of alkali were evaluated. The products have relatively low critical micelle concentrations [(6.8 X 10(-7))-(1.0 X 10(-5) mol/L)] and a wide range of surface tensions at the critical micelle concentration (gamma(cmc))values (30.3-38.0 mN/m) at 25 degrees C, depending on both the alkyl-chain length and the number of ethylene oxide (EO) units. When used alone, these nonionic surfactants are not efficient for reducing crude oil/connate water interfacial tension (IFT), but their binary mixtures with a highly hydrophobic zwitterionic surfactant, didodecylmethyl hydroxylpropyl sulfobetaine (diC(12)HSB), are very efficient in reducing Daqing crude oil/connate water IFTs. Ultralow IFTs can be achieved in a wide total concentration range (0.3-10 mM) at 45 degrees C, and the low limit (0.3 mM) can be further reduced to 0.15 mM by adding an anionic surfactant, a-olefin sulfonate (AOS), to the binary mixtures at a small mole fraction (0.1). Both the binary and ternary mixtures display good resistance against adsorption by Daqing sandstone, and when dissolved in connate water, they prefer to keep the negatively charged rock surface water-wet and produce a crude-oil in-water emulsion. In addition, the incorporation of AOS into the binary mixtures was found to be beneficial for preventing crystallization of zwitterionic surfactant and flocculation of nonionic surfactants, as reported previously. Comparatively, for SP flooding free of alkali, the Guerbet alcohol ethoxylates are as excellent as the glyceryl ether ethoxylates, nonionic surfactants with a similar double-alkyl single-head structure, but are far superior to the conventional nonionic surfactants with monoalkyl single head structures.