Structure formation of poly(ether-imide) films using vapor induced phase separation was studied in relation with the elaboration process. Dense and cellular structure can be obtained depending on the processing parameters. When the solvent evaporation took place under humidified air, kinetic data from gravimetric measurements showed that the overall mass of the film was first increasing before decreasing to a plateau. An analysis in terms of drying rate revealed that mass transfers were occurring as if there was a thin layer of volatile molecule at the top of the forming film. Solvent and non-solvent transfers were demonstrated to be closely dependent on the relative humidity.