FT-Raman, IR and near IR investigations of 2 polymorphs of olanzapine are presented, establishing the main features that allow the discrimination of these cryst. forms using vibrational spectroscopic methods. Ab initio calcns. on the basis of the d. functional theory were used to det. the stable conformations. The calcd. vibrational spectra were compared to the exptl. ones to identify the conformers corresponding to each polymorph and to assign the vibrational bands to the internal vibrations of the olanzapine mol. The authors' results support the hydrogen bonding pattern proposed by the reported cryst. structure and provide valuable information on the structural and thermodynamical relationship between the investigated polymorphs.