The polar orientation and alkyl chain conformational ordering of sodium dodecyl sulfate (SDS) at the octadecanethiol/water hydrophobic interface have been investigated using sum frequency vibrational spectroscopy (SITS). The effects of different concentrations of the cationic polymer poly(ethylenimine), PEI, along with the solution pH and temperature have been determined. PEI was found to promote synergistic adsorption of ordered SDS at concentrations as low as 10 ppm at pH 6.0 through cooperative polymer/ surfactant complex formation. At lower pH's and in the presence of the polymer stronger interfacial ordering of the surfactant was observed, attributed to increased protonation of PEI amino groups in acidic solutions and consequent decreases in the intersurfactant spacings of complexed SDS. SDS was found to be present at the hydrophobic surface at temperatures of up to 60 degreesC, although thermally induced motion of the surfactant chains resulted in a monotonic decrease in the degree of conformational order with rising temperature. In the presence of PEI the effect of temperature on the ordering of SDS, present predominantly in the form of a polymer/surfactant complex, was qualitatively similar.