Molecular modeling methods are used to estimate the influence of impurity species: water, O-2, and SO2 in flue gas mixtures present in postcombustion CO2 capture using a metal organic framework, HKUST-1, as a model sorbent material. Coordinated and uncoordinated water effects on CO2 capture are analyzed. Increase of CO2 adsorption is observed for both cases, which can be attributed to the enhanced binding energy between CO2 and HKUST-1 due to the introduction of a small amount of water. Density functional theory calculations indicate that the binding energy between CO2 and HKUST-1 with coordinated water is similar to 1 kcal/mol higher than that without coordinated water. It is found that the improvement of CO2/N-2 selectivity induced by coordinated water may mainly be attributed to the increased CO2 adsorption on the hydrated HKUST-1. On the other hand, the enhanced selectivity induced by uncoordinated water in the flue gas mixture can be explained on the basis of the competition of adsorption sites between water and CO2 (N-2). At low pressures, a significant CO2/N-2 selectivity increase is due to the increase of CO2 adsorption and decrease of N-2 adsorption as a consequence of competition of adsorption sites between water and N-2. However, with more water molecules adsorbed at higher pressures, the competition between water and CO2 leads to the decrease of CO2 adsorption capacity. Therefore, high pressure operation should be avoided in HKUST-1 sorbents for CO2 capture. In addition, the effects of O-2 and SO2 on CO2 capture in HKUST-1 are investigated: The CO2/N-2 selectivity does not change much even with relatively high concentrations of O-2 in the flue gas (up to 8%). A slightly lower CO2/N-2 selectivity of a CO2/N-2/H2O/SO2 mixture is observed compared with that in a CO2/N-2/H2O mixture, especially at high pressures, due to the strong SO2 binding with HKUST-1.