Laccases (Lac) are oxidizing enzymes with a broad range of applications, for example, in soil remediation, as bleaching agent in the textile industry, and for cosmetics. Protecting the enzyme against degradation and inhibition is of great importance for many of these applications. Polymer vesicles (polymersomes) from poly(N-vinylpyrrolidone)-block-poly(dimethylsiloxane)-block-poly(N-vinylpyrrolidone) (PNVP-b-PDMS-b-PNVP) triblock copolymers were prepared and investigated as intrinsically semipermeable nanoreactors for Lac. The block copolymers allow oxygen to enter and reactive oxygen species (ROS) to leave the polymersomes. EPR spectroscopy proved that Lac can generate ROS. They could diffuse out of the polymersome and oxidize an aromatic substrate outside the vesicles. Michaelis-Menten constants K-m between 60 and 143 mu M and turn over numbers k(cat) of 0.11 to 0.18 s(-1) were determined for Lac in the nanoreactors. The molecular weight and the PDMS-to-PNVP ratio of the block copolymers influenced these apparent Michaelis-Menten parameters. Encapsulation of Lac in the polymersomes significantly protected the enzyme against enzymatic degradation and against small inhibitors: proteinase K caused 90% less degradation and the inhibitor sodium azide did not affect the enzyme's activity. Therefore, these polymer nanoreactors are an effective means to stabilize laccase.