Ligand exchange is one of the possible synthetic routes to obtain99mTc coordination compounds. However, the success of this route depends on the availability of good precursors. The objective of this work is the preparation of the complex [99mTc(tu)6]3+ (tu = thiourea), as a potential precursor for99mTc(III) coordination compounds. The preparation was successfully performed in acidic conditions, the excess of tu serving as reducing agent. At pH values higher than 3, the compound becomes unstable and on addition of polydentate ligands new Tc(III) complexes are formed. With edta, the complex99mTc(III)-edta was obtained in high yield.