The reaction of [ReX(CO)(5)] with thiosemicarbazones H2LR derived from beta-keto esters (X = Cl, Br; R = Me, Ph) allowed the isolation of cyclic trimeric complexes [Re-3(pyz(R))(3)(CO)(9)], where pyz(R) is the pyrazolonate corresponding to the thiosemicarbazone. Electron spray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) monitoring of the reactions of H2LPh in toluene confirmed that the trimer was formed in the reaction mixture and no higher oligomer was detected. These studies, together with the X-ray structures of the trimeric complexes, afford new insight into the factors influencing the self-assembly of pyrazolonaterhenium(I) complexes.