Molecular beams have been used to search for evidence for a weakly bound molecular precursor in the interaction of O-2 with Al(111). The experiments are consistent with a precursor whose binding energy is smaller than 0.1 eV. The total reflectivity as a function of incidence angle shows a pronounced dip at 25degrees for E-trans between 90 and 300 meV. This feature corroborates an earlier observation by Osterlund in sticking measurements. Modeling using a reduced dimensionality potential energy surface shows a similar behavior which is caused by steering into a shallow molecular adsorption well located at the same site in the unit cell as the maximum in the barrier towards dissociative adsorption. This effect is not observed if the molecular adsorption well is located at the same site as the minimum energy pathway to dissociative adsorption. (C) 2003 American Institute of Physics.