The defect states in chalcogenide glasses, including positively charged, negatively charged and neutral three-fold coordinated sulphur clusters, were studied by SCF MO CNDO/2 method. By comparing energies of the clusters, net charges of atoms, bond order functions and other parameters, conclusion has been drawn which is in good agreement with that of Kastners, and thus supports his valence-alternation pairs theory. The effect of lone-pair electrons on bonding in defect states is also discussed and it is shown that they play a significant role in chalcogenide glasses.