The polyoxoanion-supported iridium complex [n-Bu4N](5)Na-3[(1,5-COD)Ir.P2W15Nb3O62], 1, was previously shown to be an active homogeneous catalyst for cyclohexene oxidation catalysis via a classic, Haber-Weiss ROO (R = 2-cyclohexen-1-yl) free-radical-chain autoxidation mechanism. The question posed in the present work is: "Is the true catalyst polyoxoanion-supported?", an important question given that polyoxoanion-supported catalysts are one of only eight new conceptual subclasses of polyoxoanions in catalysis developed over the last ca. 20 years. The present work probes the above question via a range of methods, specifically: catalytic activity and kinetic studies; isolation of the catalyst following the reaction and characterization of the catalyst by transmission electron microscopy (TEM), elemental analysis, IR, nuclear magnetic resonance, ultracentrifugation and fast atom bombardment mass spectrometry molecular weight methods, plus ion-exchange resin tests to provide evidence for inner-sphere attachment of the oxidized Irn+ moiety to the P2W15Nb3O629-polyoxoanion. The results suggest "[(n-Bu4N)(6)Na-3(HO)(3)Ir3+.P2W15Nb3O62](x) (X = 1, 2)" as a working model for the average molecular formula of the catalyst. Most importantly, the results are definitive in answering the question posed: the true catalyst is polyxoanion-supported. This finding is of significance since the autoxidation of cyclohexene beginning with [n-Bu4N](5)Na-3[(1,5-COD)Ir.P2W15Nb3O62], 1, is, historically, the first bona fide example of a polyoxometalate-supported catalyst and, hence, the first demonstration of a new subclass of polyoxoanion-based catalysts. However, a remaining challenge is the development of more important oxidations or other chemistries from polyoxoanion-supported catalysts. (C) 2002 Published by Elsevier Science B.V.