Light yield of some yttrium-doped PbWO4 crystals increased after low dose irradiation, and the radiation hardness was sensitive to annealing temperature. The radiation instability was attributed to interstitial oxygen ions. In this study, the effect of melt stoichiometry on radiation instability was investigated. The results show that an excessive WO3 in the starting materials can significantly improve the radiation stability of PWO:Y3+. But an excessive PbO in the melt deteriorated the radiation stability. The extra amount of lead vacancies prevented the defect (Y-Pb(3+))' from forming [2(Y-Pb(3+)) '-O"(i)] cluster, thus improving the radiation stability of PWO:Y3+.(C) 2003 Elsevier B.V. All rights reserved.