Novel complexes between methyltrioxorhenium (MTO) and natural polysaccharides like chitin and chitosan, or modified chitosan-bearing N-substituted acyl or alkyl pyridine moieties, were synthesized and used for the oxidation of alkenes, including acid-sensitive styrene derivatives, with urea hydrogen peroxide adduct (UHP) as primary oxidant. High conversions and yields of the corresponding epoxides were obtained. Silica–chitosan hybrid composites with increased surface area and mechanical properties were also prepared and used as MTO supports. A correlation on the role played by the different MTO-coordinating ligand sites of supports, on the experimental outcome of olefin catalytic oxidations, is discussed.