The present invention relates to novel crystalline solvates of N-[2-[(1S)-1-(3-ethoxy-4-methoxyphenyl)-2-(methylsulfonyl)ethyl]-2,3-dihydro-1,3-dioxo-1 H-isoindol-4-yl]acetamide and methods for their preparation. Furthermore, the invention concerns pharmaceutical compositions comprising an effective amount of one or more of the novel crystalline solvates and the use thereof as medicaments.