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From pure powders of forms I and II of phenobarbital, X-ray diffraction patterns were recorded at room temperature. The starting crystal structural models were found by a MonteCarlo simulated annealing method. The structures of the two forms were obtained through Rietveld refinements. Soft restraints were applied on bond lengths and bond angles, all H-atom positions were calculated. The cell of form I is monoclinic with the space group P21/n, Z = 12, Z' = 3. Form II has a triclinic cell, with the space group P&macr1;, Z = 6, Z' = 3. For both forms, the crystal cohesion is achieved by networks of N-H…O hydrogen bonds along [101]. The broadening of the Bragg peak profiles is interpreted in terms of isotropic strain effects and anisotropic size effects. [ABSTRACT FROM AUTHOR]