Rhodium(III) hydroxide was shown previously to possess the catalytic activity in the rearrangement reaction of primary oximes to amides. Alkaline solutions of rhodium(III) chlorocomplexes are routinely used for Rh(OH)3 preparation and allow for a diversity of rhodium hydroxocomplexes that hold a wide range of catalytic properties. In this study, two Al2O3 supported catalysts containing 1) exclusively [Rh(OH)6]3− species and 2) a mixture of rhodium hydroxocomplexes with one to four rhodium atoms ([Rh(OH)6]3–, [Rh2(μ−OH)2(OH)8]4–, [Rh3(μ−OH)3(OH)12]6–, [Rh4(μ−OH)6(OH)12]6) were prepared, characterised (XRD, Raman, EXAFS) and tested in the benzaldoxime to benzamide rearrangement. The catalyst containing polynuclear complexes possessed at least three times higher turnover frequency (TOF) and better selectivity than the [Rh(OH)6]3– based catalyst. According to the EXAFS data, the reduction of rhodium(III) took place under reaction conditions yielding small rhodium clusters (CNRh-Rh = 4), which were proposed to be the active sites of catalysts.