The atomic structure of 1/2 monolayer of Al on GaAs(110) is examined by low energy electron diffraction (LEED) intensity analysis. Room temperature deposition produces a disordered overlayer that does not modify the substrate structure. However, an ordered 1×1 structure results when the system is annealed at 450°C. The AES and LEED measurements suggest a reaction of Al on GaAs to form AlAs, in agreement with some of the photoemission data. As a preliminary result, the multiple scattering analysis indicates that a structure for which Al replaces the second layer Ga provides the most satisfactory description of the measured LEED intensities.