The reactions of methylacetoacetate and ethyl 2-methylacetoacetate thiosemicarbazones (H2LA and H2LB, respectively) with [ReX(CO)(5)] and [ReX(CO)(3) (CH3CN)(2)] (X = Cl, Br) were explored under various experimental conditions. Besides the adducts fac-[ReX(CO)(3)(H2L)], in which the rhenium is coordinated to three carbonyl groups, the X anion, and the N,S-bidentate thiosemicarbazone ligand, the following complexes were also isolated: fac-[ReBr(CO)(3)(Hpyz (B))], the tetrameric complexes fac-[Re(pyZ(A)) (CO)(3)](4) and fac-[Re(pyZ(B)) (CO)(3)](4), and fac-[Re(pyZ(B)) (CO)(3)(H2O)] (where HpyZ(A) and Hpyz(B) are pyrazolones derived by cyclization of H2LA and H2LB, respectively). The cyclization reactions were monitored by H-1 NMR spectroscopy and the complexes isolated were identified by elemental analysis, mass spectrometry, IR and H-1 NMR spectroscopy, and in some cases by X-ray diffractometry. The isolation and the full structural identification of the rather unusual fac-[ReBr(CO)(3)(HpyZ(B))], which contains the enol form of the pyrazolone ligand, affords new insight into the cyclization of thiosernicarbazones derived from beta-keto esters.