Disclosed is a process for preparing an anhydrous, crystalline alpha form of thalidomide, comprising cyclising N-phthaloyl-glutamine (optionally in the presence of a coupling agent and/or catalyst) in an organic solvent system and isolating the anhydrous, crystalline alpha-form of thalidomide, wherein the organic solvent system comprises a solvent selected from the group comprising straight chain or branched aliphatic ketones (such as acetone, butanone or mixtures thereof), aliphatic nitriles (such as acetonitrile, propionitrile or mixtures thereof), ethers (such as tetrahydrofuran (THF), tertiary butyl methyl ether (TBME) or mixtures thereof) and mixtures thereof, wherein the reaction mixture is heated to a temperature (optionally at reflux) between about 50°C and about 100°C, and wherein the reaction mixture is further cooled in order to isolate the anhydrous, crystalline alpha form of thalidomide, wherein the X-ray diffractogram of the anhydrous alpha form of thalidomide contains characteristic peaks at about 11.30, 14.29, 19.15, 22.82, 26.10 and 30.32 ±0.2 degrees 2-theta. Also disclosed is a process for preparing crystalline form alpha of thalidomide, comprising dissolving thalidomide in dimethylsulfoxide (DMSO), adding the mixture to methanol containing suspended seed crystals of the alpha form of thalidomide, and isolating the pure, anhydrous, crystalline c-form of thalidomide, wherein the X-ray diffractogram of the anhydrous alpha form of thalidomide contains characteristic peaks at about 11.30, 14.29, 19.15, 22.82, 26.10 and 30.32 ±0.2 degrees 2-theta.
