FTIR-attenuated total reflection (ATR) spectroscopy was used to characterize the surface excess and molecular orientation of sodium dodecylbenzenesulfonate (SLABS) adsorbed at the Al2O3/water interface. Gibbs’ surface excesses were determined in the presence of aqueous solutions ("in situ") by IR-ATR and confirmed by the solution depletion method. An in situ linear dichroism (LD) study was made of the orientation of the alkyl chains, the phenyl rings, and the SO3- head groups of SLABS adsorbed onto Al2O3. Alumina was present as a thin film (150-220 nm) on the surface of ZnSe IR internal reflection elements, allowing in situ IR-ATR adsorption measurements from the near-IR to 950 cm(-1). Analysis of the adsorption densities showed two monolayers of SLABS adsorbed in the presence of 0.15 M NaCl and ca. 1.6 monolayer in its absence, conforming to literature values. Under conditions giving an average of two monolayers coverage, LD studies revealed an average tilt angle between the phenyl ring pare-axis and the surface normal of 42.0 +/- 1.6 degrees at 28 degrees C (48.0 +/- 2.1 degrees at 4 degrees C) at pH 3.8, and 47.3 +/- 1.9 degrees at 28 degrees C (49.5 +/- 1.5 degrees at 4 degrees C) at pH 6.6. Under the same conditions, alkyl chains in adsorbed SLABS exhibited an ordered packing with a tilt angle of ca. 45 +/- 2.5 degrees at 4 degrees C and either pH, with free rotation of the chain about the extended chain axis. At the lower temperature, decreased molecular motion allows the increased packing density observed. Calculated tilt angles for the SO3- changed markedly with solution pH, presumably due to hydrogen bonding with the hydrated Al2O3 surface.