The S-Se exchange during the hydrode-selenium (HD-Se) reaction of selenophene was investigated over MoS2/Al2O3 and intrazeolite Mo sulfide clusters. The local structure of Se incorporated into the Mo sulfide catalysts was studied by means of Se K-edge XAFS. It was found that the capacity of S-Se exchange during the HD-Se reaction and dynamic behavior of Se incorporation strongly depend on the cluster size of Mo sulfides. (C) 2003 Elsevier Science B.V. All rights reserved.