The equilibrium geometry, electronic structure, and relative stability of titanozeolites have been investigated at an ab initio level by using the periodic quantum mechanical program CRYSTAL. Nine periodic crystalline models of composition (TiO2)(x)(SiO2)(1-x), with different framework type structures (chabazite, sodalite, and alpha-quartz) and values of x, have been considered. Rutile has also been considered as a TiO2 reference system. The geometry of the various models has been fully optimized by using a split valence double-zeta basis set. The effect of more extended basis sets on the relative stabilities and electronic properties at the optimized geometry has been explored. Four-fold-coordinated titanium, as present in titanozeolites, is 17-24 kcal.mol(-1) less stable than 6-fold-coordinated Ti in rutile, the actual value depending on the structure and composition of the models.