Gold modified nanoporous silica based magnetic microparticles have been prepared as support for the immobilization of the enzyme horseradish peroxidase (HRP). The enzyme modified gold microparticles were retained onto the surface of a solid carbon paste electrode with the help of a permanent magnet. The analytical performances of the resulting biosensor were characterized by studying hydroquinone (HQ) and hydrogen peroxide. The former was monitored by the direct electroreduction of the biocatalytically generated quinone. Several experimental parameters influencing the biosensor response were investigated. A linear response to HQ was obtained in the concentration range comprised between 5 Â 10 À7 and 4.5 Â 10 À6 M with a detection limit of 4 Â 10 À7 M. The enzyme electrode provided a linear response to hydrogen peroxide over a concentration range comprised between 5 Â 10 À7 À 1.3 Â 10 À4 M with a detection limit of 4 Â 10 À7 M. The inhibition of the biosensor response in the presence of thiols e.g. cysteine, captopril, glutathione and Nacystelyn (NAL) has been pointed out.