Rhenium carbonyls bonded to dealuminated zeolite Y calcined at 300 and at 500 degreesC were tested as catalysts for hydrogenation of ethylene and of propylene and characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies, even under catalytic reaction conditions. The initially formed rhenium tricarbonyls bonded to the zeolite were converted into rhenium dicarbonyls during catalysis, as shown by IR and EXAFS spectra, and the coordination of the Re to the support changed, as shown by EXAFS results. Consequently, there were bonding positions on the Re centers for reactive ligands, including those identified by IR spectroscopy as alkyls and pi-bonded alkenes. The catalysis is suggested to proceed via reaction of alkyl and hydride ligands on rhenium centers incorporating two CO ligands. (C) 2003 Elsevier B.V. All rights reserved.