Boom clay formation, a deposit of slightly over-consolidated marine clay that belongs to the Oligocene series in the north east of Belgium, has been studied as a possible host material of nuclear waste disposal. In this context, the long-term deformation behavior of Boom clay is of crucial importance in the performance assessment of the whole storage system. In this study, low and high pressure oedometer tests are carried out; the e-log σ′v (void ratio–logarithm of vertical effective stress) and e-log t (void ratio–logarithm of time) curves obtained are used to determine the compression index Cc*, swelling index Cs* and secondary deformation coefficient Cα during both loading and unloading. The relationship between Cα and the effective stress ratio (σ′v/σ′c, vertical effective stress to pre-consolidation stress) is analyzed, and it is observed that Cα increases linearly with log σ′v/σ′c. Examination of the ratio of Cα/Cc* for various soils shows that the secondary deformation behavior of Boom clay is similar to that of shale and mudstone. The relation between Cα and Cc* is linear; but the relation between Cα and Cs* is bi-linear. The bi-linearity observed is related to two different mechanisms: the mechanically dominated rebounding and the physico-chemically dominated swelling.