The penetration of vapors of pure and mixed solvent into ordered thin films of symmetric, diblock copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA) was investigated by neutron reflectivity. In the case of cyclohexane, a marginal solvent for PS but a poor solvent for PMMA, only the PS layer at the surface was swollen by the cyclohexane. The underlying multilayered structure was unperturbed by the solvent vapors, which indicates that the first PMMA layer near the surface, even though it has a thickness of only similar to 150 Angstrom acts as an effective diffusion barrier preventing penetration of the solvent. In the case of a mixed solvent, in particular a 75/25 methanol/toluene mixture, where toluene is a good solvent for both components, the entire multilayer is swollen. However, the swelling of the layers is not uniform as a function of depth. In addition, the appearance of second-order Bragg reflections shows that the swelling of the PS and PMMA layers is not equal.