We have investigated the structural changes in thin films of lamellar poly(styrene-b-butadiene) diblock copolymers during treatment with saturated cyclohexane vapor, it solvent slightly selective for polybutadiene. Using real-time, in situ grazing-incidence small-angle X-ray scattering (GISAXS), the swelling and the rearrangement of the lamellae were investigated with it time resolution of a few seconds, and the Underlying processes oil the molecular level were identified. After it few minutes in vapor, a transient state with higher degree of lamellar order and orientation was encountered. Additional parallel lamellae formed which we attribute to the increased degree of coiling of the polymers in the swollen state. Eventually, the film became disordered. These changes are attributed to the increased mobility of the swollen polymers,and the gradually decreasing segment-segment interaction parameter in the film its solvent is absorbed.