Pigmentary titanium dioxide is doped with aluminium during manufacture by the chloride process with the dual aims of promoting the phase transition from anatase to rutile and reducing the photocatalytic activity of the particles. To define the structure of aluminium-doped anatase and rutile and to determine the effect of doping on the properties of titania these systems were studied by the semi-empirical quantum chemical method MSINDO. A series of models for aluminium-doped anatase and rutile, which contained aluminium atoms in substitutional and interstitial sites and combinations of substitutional aluminium atoms and interstitial aluminium and titanium atoms and oxygen vacancies, was compared. The positions of atoms close to the defect positions were allowed to relax. This was found to have a large stabilization effect. From a comparison of the calculated defect formation energies the most likely forms of aluminium-doped anatase and rutile were determined. The most stable defect structure for anatase was found to be obtained by the replacement of a Ti–O–Ti unit by two Al atoms separated by an oxygen vacancy. The second most stable anatase defect structure was found to be obtained by the simple replacement of a Ti atom by an Al atom. This replacement leads to the oxidation of a neighboring O atom. In rutile these defect structures were also found to be the most stable, but in the reverse order. Al-defects involving the interstitial placement of Al or Ti are significantly less energetically favourable for both anatase and rutile.