The formation of polymer films produced by the phase separation process occurring when a cast poly(etherimide)/N-Methyl-2-Pyrrolidone solution was exposed to humid air was studied. It was found that above a relative humidity value of 27%, the films presented a cell-like structure. The size of the cells was shown to decrease when the relative humidity increased. This effect was more pronounced at the film/substrate interface than near the surface. A cell-size gradient from one face of the film to the other was also clearly observed. A phenomenological model has been proposed to explain the morphology obtained by a phase separation induced by the water vapor in the studied system, taking into account thermodynamics and kinetics considerations. In this model, the cell-like structure setting up is shown to result from a nucleation and growth process accompanied by a coalescence coarsening. It was illustrated by a composition path on the ternary phase diagram. It was shown how the relative humidity influenced the film composition leading to the preferential nucleation compared to the growth and coalescence of the cells. Finally, it was found that the cell-size anisotropy resulted in the solvent and non-solvent mass transfers in the film, bringing to the fore the determining role of kinetics.