Time-of-flight detection is used to measure the resonant neutralization probabilities for 1.2 and 2.0 keV 7Li+ ions singly scattered from Cs and Al atomic sites of the Cs/Al(100) surface. For the lowest Cs coverages, neutral fractions for scattering from Cs are ∼0.70 and ∼0.78, while the neutral fractions for scattering from Al are ∼0.43 and ∼0.55, respectively. The disparity in neutral fractions between scattering from Cs and Al demonstrates that the surface local electrostatic potential is significantly inhomogeneous. At higher Cs coverages [≳0.05 monolayer (ML)], however, the neutral fractions for scattering from Al and Cs sites become nearly equal, which indicates that the variations in the surface potential are greatly reduced. For Cs coverages above 0.075 ML, the formation of Li− negative ions via resonant tunneling is observed for scattering from Cs.