The bioavailability limitations of proteins make them difficult to be directly delivered, particularly in diseases caused by insufficient amounts or inactive variants of those proteins. Nanoreactors represent a new promising approach to overcome these limitations because they serve both to protect the protein in their aqueous interior, and simultaneously to allow the protein to act in situ. Here we examine an antioxidant nanoreactor based on SOD encapsulated in amphiphilic block copolymer nanovesicles, and analyze its behavior as a function of the copolymer composition. The membrane of the triblock copolymer nanovesicles plays a double role, both to shield the sensitive protein and selectively to let superoxide and dioxygen penetrate to its inner space. The encapsulation efficiency for different triblock copolymer vesicles was quantified by fluorescence correlation spectroscopy using a fluorescently labeled SOD. Pulse radiolysis experiments and an enzymatic assay were used to compare the permeability of the wall-forming membranes towards superoxide anions. While the encapsulation efficiency mainly depends on the vesicle dimensions, the membrane permeability is mainly affected by the length of the hydrophobic PDMS middle blocks of our polymers. For polymers with very long PDMS chains superoxide anion transport across the membranes was too slow to be detected by our experiments.