Dissociative chemisorption of O-2 on Cu(100), S/Cu(100) and Ag/Cu(100) surface alloy has been investigated by Auger electron spectroscopy (AES). A strong reduction in the initial O-2 chemisorption probability (S-0) from 0.05 to 7.4 x 10(-3) is observed already at an Ag coverage of 0.02 ML. Further Ag deposition results only in a moderate decrease in So. Similar inhibition of O-2 dissociation is observed on S/Cu(100). It is concluded that at very low Ag coverages, the reduced reactivity of Ag/Cu(100) towards O-2 dissociation is primarily due to the steric blocking of the surface defects and that any electronic effects are only secondary and present only at higher Ag coverages.