The effects of adsorbed carbon monoxide and sulfur on the electrochemical properties of Ni(111) and Ni(110) in LiClO4-poly(ethylene oxide) (PEO) solutions have been examined by cyclic voltammetry at a temperature of 333 K in ultrahigh vacuum (UHV) environments. Rather sharp peaks were found in the first scan in the negative direction for c(4 x 2)CO and sulfur overlayers on Ni(111) centered at 0.5 and 1.25 V, respectively. In contrast, the voltammetric features observed in similar experiments for Ni(110) exposed to 30 L of CO and for Ni(110)-c(2 x 2)S were broad and not as well defined. These processes are attributed to reactions between Li underpotential deposition (UPD) and CO and sulfur overlayers to yield, correspondingly, Li2O (and a polymeric Li-CO adduct) and Li2S as the most likely products.