Phenylethanol dehydrogenase was isolated from Lactobacillus kefir, and some kinetic parameters and properties of native and immobilised phenylethanol dehydrogenase on the modified DEAE-Cellulose, Bioran, Mobitec, and VA-epoxy were investigated. The activity of immobilised phenylethanol dehydrogenase was examined in a glow-injection-analysis system in relation to the concentration of diacetyl in water and beer.