The impact of humidity on PIM1 membrane performance, i.e. CO2 permeability and CO2/N2 selectivity, is investigated in this study. While most novel materials are tested in dry conditions, flue gas contains a significant amount of water that could affect dramatically the membranes and the productivity of the separation process. In this work, mixed gas permeability was measured by a flow method measurement with humidity and CO2 sensor. Three different concentrations of CO2 are studied, pure CO2, and mixtures close to coal-fired and gas-fired power plants. Increasing water activity in the feed stream induces a decrease in the permeability of both CO2 and N2. This behavior is related to the competitive sorption between water and CO2, and to the formation of water clusters which hindered the transport of others gas species. The same behavior is observed also for aged membranes.