We report the fluorescence decrease of the water-soluble pi-pi-conjugated polymer poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene, MPS-PPV) by the catalytic activity of horseradish peroxidase in the presence of H2O2. MPS-PPV acts as a donor substrate in the catalytic cycle of horseradish peroxidase where the electron-deficient enzymatic intermediates compounds I and II can subtract electrons from the polymer leading to its fluorescence decrease. The addition MPS-PV of phenolic drug acetaminophen to the former solution favors the decrease of the polymer fluorescence, which indicates the peroxidase-catalyzed co-oxidation of MPS-PPV and acetaminophen. The encapsulation of horseradish peroxidase within polyacrylamide microgels allows the isolation of intermediates compound land compound II from the polymer, leading to a fluorescence decrease that is only due to the product of biocatalytic acetaminophen oxidation. This system could be used to develop a new device for phenolic compounds. detection.