By using the electropositive Cs and Li to induce changes in the surface work function ϕ, the positive secondary ion emission is shown to vary exponentially with ϕ, in close resemblance to negative secondary ion emission. This is in contrast to the enhancement effect of adsorbed oxygen on positive secondary ion emission which shows no correlation with the work function change. This suggests that the oxygen enhancement effect originates mostly from surface chemical changes rather than from the formation of a surface electric dipole layer. © 1983, American Vacuum Society. All rights reserved.