The real time changes occurring within films of cylinder-forming poly(alpha-methylstyrene-block-4-hydroxystyrene) (P alpha MS-b-PHOST) were monitored as they were swollen in tetrahydrofuran (THF) and acetone solvent vapors. In situ information was obtained by combining grazing incidence small-angle X-ray scattering (GISAXS) with film thickness monitoring of the solvent vapor swollen films. We show that for self-assembly to occur the polymer thin film must surpass a swollen thickness ratio of 212% of its original thickness when swollen in THF vapors and a ratio of 268% for acetone vapor annealing. As the polymer becomes plasticized by solvent vapor uptake, the polymer chains must become sufficiently mobile to self-assemble, or reorganize, at room temperature. Using vapors of a solvent selective to one of the blocks, in our case PHOST-selective acetone, an order-order transition occurred driven by the shift in volume fraction. The BCC spherical phase assumed in the highly swollen state can be quenched by rapid drying. Upon treatment with vapor of a nonselective solvent, THF, the film maintained the cylindrical morphology suggested by its dry-state volume fraction. In situ studies indicate that sell-assembly occurs spontaneously upon attaining the threshold swelling ratios.