Density functional theory calculations are carried out for Rh(111)-p(2×2)-CO, Rh(111)-p(2×2)-S, Rh(111)-p(2×2)-(S+CO), Rh(111)-p(3×3)-CO, Rh(111)-p(3×3)-S and Rh(111)-p(3×3)-(S+CO), aiming to shed some light on the S poisoning effect. Geometrical structures of these systems are optimized and chemisorption energies are determined. The presence of S does not significantly influence the geometrical structure and chemisorption energy of CO and vice versa, which strongly suggests that the interaction between CO and S on the Rh(111) surface is mainly short-range in nature. The long range electronic effect for the dramatic attenuation of the CO methanation activity by sulfur is likely to be incorrect. It is suggested that an ensemble effect may be dominant in the catalytic deactivation.