Tablets made of different phenobarbital modifications were examined for their specific compression behaviour and tablet characteristics. Among the thirteen reported polymorphic forms of phenobarbital modifications I, II and III were chosen to prepare tablets. The amorphous form was also produced and subsequently used as the starting material for one of the crystallographic forms. All forms were analysed for identity and purity using hotstage microscopy, IR, DTA, DSC, and X-ray diffraction. Samples of the various forms were stored for certain periods of time under defined conditions to check their stability. The compressional behaviour of the three modifications (compression energy and densification according to Heckei) was examined. Despite previous reports, no change in polymorphic forms was observed during compression. Crushing forces, bending and radial tensile strengths as well as the stiffness of the tablets were measured with a newly developed apparatus. Modification III proved to be superior with respect to both the compression behaviour and the mechanical properties of the tablets.