Direct electron transfer between an immobilized biological compound and an electrode is one of the most interesting transduction processes for the development of fast responding amperometric biosensors. Different biocatalysts like horseradish peroxidase, cytochromec , myoglobin, microperoxidase MP-11 and haemin, all of them catalyzing the reduction of H 2 O 2 , have been investigated aiming on their ability for direct electron-transfer reactions when covalently tethered to self-assembled monolayers (SAMs) on gold. As direct electron-transfer processes are predominantly limited by the distance between the active site of the biocompound and the electrode surface, the highest electrocatalytic efficiency with the monolayer-immobilized biocatalysts was observed for the smallest peroxidase-active compounds (e.g., microperoxidase MP-11, haemin). Although these compounds show a significant lower catalytic activity for the reduction of H 2 O 2 in homogeneous solution, the catalytic activity of horseradish peroxidase is by a factor of 3300 higher as compared with that of haemin. Haemin exhibits a more than tenfold higher electrocatalytic activity when immobilized at a monolayer. This tremendous difference between the catalytic activity in homogeneous solution and the electrocatalytic activity of the monolayer-immobilized biocatalyst could be attributed to a higher surface concentration for the smaller compounds, the improved access for the substrate to their active sites and, most significantly, the increased electron-transfer rate due to the decrease of the distance between redox site of the biocatalyst and electrode surface. Hence, for the development of enzyme electrodes based on direct electron-transfer processes between monolayer-immobilized biocatalysts and the electrode the size of the biocatalyst itself should be decreased. Such catalytically-active compounds with decreased protein shell have been called minimized enzymes or minizymes .