A new amperometric hydrogen peroxide enzyme inhibition biosensor for the indirect determination of toxic mercury ions, Hg2+, based on catalase immobilized on a glassy carbon electrode surface by cross linking with glutaraldehyde and bovine serum albumin, is reported. The parameters influencing biosensor performance were optimized, including enzyme loading, the amount of hydrogen peroxide, the applied potential and electrolyte pH. It was shown that the inhibition of catalase by Hg2+ species is irreversible, with a linear inhibition response between 5 x 10(-11) and 5 x 10(-10) M. The limit of detection calculated as 10% inhibition was 1.8 x 10(-11) M and is the lowest reported until now. Electrochemical impedance spectroscopy was successfully used as a diagnostic of inhibition. Interferences from other heavy metal ions and organic pesticides were evaluated and the inhibition showed very good selectivity towards Hg2+. The method was successfully applied to the determination of mercury ions in different types of water sample. (C) 2017 Elsevier B.V. All rights reserved.