Production of crystalline forms (specifically polymorphic forms or solvates) of enantiomers of modafinil (A) involves dissolving an enantiomer of (A) (preferably the levorotatory isomer, (-)-(A)) in a solvent other than ethanol, crystallizing the enantiomer and recovering the obtained crystalline form. Some polymorphic forms and solvates of (-)-(A) are new. Independent claims are included for: (a) the following new polymorphic forms of (-)-(A): (i) Form II, having an X-ray diffraction pattern with maxima corresponding to interplanar spacings of 8.54Å, 7.57Å, 7.44Å, 4.56Å, 3.78Å and 3.71Å； (ii) Form III, having an X-ray diffraction pattern with maxima corresponding to interplanar spacings of 12.58Å, 8.54Å, 5.01Å, 4.10Å, 3.97Å and 3.20Å； and (iii) Form IV, having an X-ray diffraction pattern with maxima corresponding to interplanar spacings of 12.38Å, 8.58Å, 7.34Å, 5.00Å and 4.09Å； (b) a new dimethyl carbonate solvate of (-)-(A), having an X-ray diffraction pattern with maxima corresponding to interplanar spacings of 12.31Å, 9.69Å, 9.09Å, 8.54Å, 7.27Å, 6.21Å, 5.45Å, 5.10Å, 5.00Å, 4.83Å, 4.63Å, 4.46Å, 4.22Å, 4.21Å, 4.09Å, 3.78Å, 3.62Å, 3.53Å, 3.42Å, 3.32Å, 3.24Å, 3.21Å and 3.10Å； (c) a method for converting a first crystalline form of an enantiomer of (A) (specifically Form I of (-)-(A)) into another crystalline form, involving suspending the first crystalline form in a suitable solvent and recovering the second form (the solvate specifically being acetonitrile, with the product being recovered as acetonitrile solvate)； (d) a new acetonitrile solvate of (-)-(A), having an X-ray diffraction pattern with maxima corresponding to interplanar spacings of 16.17Å, 14.14Å, 12.32Å, 10.66Å, 9.79Å, 9.29Å, 8.54Å, 8.15Å, 7.80Å, 7.09Å, 6.31Å, 5.83Å, 5.62Å, 5.41Å, 5.10Å, 4.90Å, 4.66Å, 4.58Å, 4.46Å, 4.33Å, 4.20Å, 4.02Å, 3.92Å, 3.835Å, 3.72Å, 3.60Å, 3.57Å, 3.45Å, 3.33Å, 3.24Å, 3.19Å, 3.0Å and 3.03Å； (e) the preparation of optically active (A) from modafinic acid (B) by: (i) optically r
