Solvent vapor annealing (SVA) is frequently used to improve the ordering in diblock copolymer thin ﬁlms. An important question is which SVA protocol should be chosen to ensure thermodynamic equilibrium. Here, we investigate two thin ﬁlms from a low molar-mass, lamellae-forming polystyrene-block-polybutadiene (PS-b-PB) diblock copoly-mer (28.0 kg/mol). The ﬁlms are prepared by spin-coating Si wafers from toluene solutions and have ﬁlm thicknesses of 215 nm and 332 nm. The as-prepared ﬁlms have mainly the parallel lamellar orientation with a lamellar thickness Dlam,par signiﬁcantly lower than in the bulk. SVA cycles were carried out with cyclohexane, and the structural changes were followed in-situ using time-resolved grazing-incidence small-angle X-ray scattering (GISAXS). Before and after SVA, Dlam,par is signiﬁcantly lower than in the bulk, i.e. the equi-librium value of Dlam,par in thin ﬁlm geometry is different from the bulk value. Whereas the behavior of Dlam,par is different for the two ﬁlms in the early stages of the ﬁrst swelling, it is very similar in the late stages of swelling and during drying. During the ﬁrst drying, the lamellae deswell, initially slowly and later, when PS becomes glassy again, afﬁnely. During the second SVA cycle on the thin ﬁlm, the scaling behavior of the lamellar thickness is identical to the one during the ﬁrst drying and to the drying behavior of the thicker ﬁlm. We conclude that one cycle of solvent vapor treatment with a degree of swelling of ca. 1.5 is sufﬁcient to bring the PS-b-PB thin ﬁlms studied into equilibrium and to create a nearly defect-free lamellar structure.