The metastable zone width studies were carried out for various temperatures for supersaturated aqueous solutions of ammonium dihydrogen orthophosphate added with 1 mol% of thiourea. The metastable zone width is increased with the addition of thiourea. The induction period was studied and various critical nucleation parameters were calculated based on the classical theory for homogeneous crystal nucleation. The induction period is increased with the increase of thiourea addition. The critical nucleation parameters vary with increase in doping concentration. The results are reported and discussed. The crystals were grown for various doping concentrations of thiourea and the morphological changes are discussed. (C) 2002 Published by Elsevier Science B.V.