An organometallic lead precursor, represented by following formula:L.sub.x.Pb(THD).sub.2 [I]wherein L is an electron donor ligand selected from the group consisting of NR.sub.3 (R=H, CH.sub.3) gas and Cl.sub.2 gas; THD denotes 2,2',6,6'-tetramethyl-3,5-heptanedione; and x is in the range of 0.5 to 2, is prepared by flowing a gas phase electron donor into a bubbler containing bis (2,2',6,6'-tetramethyl-3,5-heptanedione)Pb at a predetermined temperature, to synthesize, in-situ, an adduct. The precursor exhibits a remarkable improvement in volatility, and in stability at the vaporization point.Lead-titanium based thin films prepared from the precursor, display superior reproducibility and reliability.