Carbonyl perchlorates [M(ClO 4 )(CO) 5 ] (M = Tc, Re) were prepared by treatment of [TcI(CO) 5 ] and [ReCl(CO) 5 ], respectively, with AgClO 4 in dichloromethane. The complexes were isolated in the form of yellowish and colorless crystals, respectively, and characterized by IR spectroscopy and single crystal X-ray diffraction (XRD). Perchlorate ion is coordinated to the central atom in the monodentate fashion. Perchlorate ligand in the coordination sphere of [M(ClO 4 )(CO) 5 ] (M = Tc, Re) can be readily substituted by a molecule of a coordinating solvent (acetonitrile). [Re(CO) 5 (CH 3 CN)]ClO 4 is stable with respect to thermal decarbonylation and was isolated and characterized by single crystal XRD. The coordination polyhedron of rhenium in [Re(CO) 5 (CH 3 CN)]ClO 4 is less distorted than that in [M(ClO 4 )(CO) 5 ]. [Tc(CO) 5 (CH 3 CN)]ClO 4 , contrary to its rhenium analog, is unstable in acetonitrile solution and undergoes rapid decarbonylation to form [Tc(CO) 3 (CH 3 CN) 3 ]ClO 4 .