The film formation by the phase inversion process induced by humidified air was investigated at 40°C from a 16 wt-% poly(ether imide)/N-methyl-2-pyrrolidinone solution. Weight measurements of the film forming system and video recording of the phase separation were performed in-situ to get a deep insight of the mass transfer and of the mechanism of the structure formation. Films with a cellular morphology were produced for a processing relative humidity higher than 30% and dense homogeneous films below this value. The influence of the relative humidity on the film morphology (dense or cellular, cell size, anisotropy) was discussed in terms of thermodynamic (ternary phase diagram) and kinetic aspects of the phase separation. On this basis, we propose a phenomenological model to explain what happens during the film formation.