Abstract: Described is an amperometric biosensor in thick film technology for the detection an/or determination of substances undergoing enzyme catalyzed reactions, especially glucose or lactate, comprising an inert carrier material, at least one transducer layer exhibiting a specific electrical conductivity &khgr;>104 &OHgr;−1 cm−1, and at least one bioactive layer with diffusion barrier function containing at least one enzyme specifically reacting with said substance to be measured, said bioactive layer exhibiting a specific electrical conductivity &khgr;<1 &OHgr;−1 cm−1 and said bioactive layer further effectively hinders other electroactive substances to reach the transducer layer within the time of measurement.