We report on the use of block copolymer micelles of polystyrene-b-poly( acrylic acid) ( PS-b-PAA) as matrixes for incorporating dibenzyl diselenide. We found that the water-insoluble diselenide, after being incorporated into the micelles, demonstrates glutathione peroxidase ( GPx) activity in water. Surprisingly, the mimicking system can be adjusted to show higher GPx activity by increasing the ionic strength of the solution simply upon addition of NaCl. Moreover, dibenzyl diselenide incorporated into the micelles is quite stable and maintains its GPx activity even after exposure to the atmosphere.