Essentially molecular rhodium catalysts were made from Rh(C2R4)(2)(acetylacetonate) on zeolite HY and on MgO and characterized by infrared and X-ray absorption spectroscopies. The supported rhodium species anchored to the zeolite, initially in the form of Rh(C2H4)(2), selectively catalyzed ethene dimerization, typically at 298 K and 1 bar, but when the catalyst was poisoned by CO, or the support was changed to MgO or zeolite NaY, or the rhodium was converted into small clusters, the ethene underwent predominantly hydrogenation. The preciseness of the synthesis of the supported rhodium species facilitated determination of structure-catalyst performance relationships that led to a schematic representation of how the dimerization proceeds by a mechanism involving both the rhodium complexes and zeolite surface OH groups. The reaction is facilitated by H-2 and proceeds as one ethene molecule is activated by an isolated rhodium complex and another by a weakly acidic Si-OH-Al group. (C) 2013 Elsevier Inc. All rights reserved.