The system (1- x)PbTiO₃ -(x) BiAlO₃ has been studied with regard to its structure and phase transition behaviour using x-ray, neutron and dielectric measurements. The structure is tetragonal within the solid solubility limit (x < 0:25). Interatomic distance analysis revealed that the Pb-O bond lengths remain unaffected and the only the Ti-O lengths are altered by BiAlO₃ substitution. The results are suggestive of a crossover from and A-site & B site driven ferroelectric system for x = 0 to a dominant A-site driven ferroelectric system for higher x. This cross-over is brought about by (i) a reduction in the contribution to the ferroelectric stability from B-site cations due to dilution of the Ti-sublattice by Al and (ii) a reinforcement to the stability of the ferroelectric state by the A-site cations by the Bi⁺³ cations.