The dopant compensation mechanism in 2-5 mol.% niobium (Nb)-doped Pb(Zr0.52Ti0.48) O-3 (PZT) thin films was studied to obtain single phase compositions. Transmission electron microscopy showed that the Nb-doped composition batched according to the Ti vacancy model, was single phase. This indicated that niobium donors were compensated by titanium vacancies at the B site. The beneficial effect of donors in single phase composition and detrimental effects of grain boundary second phases and accepters on the leakage currents are reported. The single-phase Nb-doped (5%) PZT thin film exhibited a low leakage current of 1 x 10(-7) A cm(-2) at 1.2 MV cm(-1).