The complexation reactions of the 99mTc(I)-(CO)3(OH2)3+precursor with tris-hydroxymethylenephosphine (THP) and a diphosphine ligand analogue (HMPE) were studied at tracer levels. Results demonstrate rapid formation of 99mTc(I)(CO)3+complexes with both THP and HMPE. In the presence of excess THP, conversion of initial complexes to stable, well-defined 99mTc species occurs indicating coordi-nation of phosphine groups to Tc(I) at all three sites trans to the carbonyl ligands.