A method for the determination of mercury(II) ions at trace levels is described. The method is based on the profound inhibitory effect of mercury on the enzyme urease. The activity of the enzyme was determined by the rate of ammonia produced from urea as followed by an ammonia gas sensitive iridium thin metalfilm-oxide-semiconductor (IrTMOS) structure. Two systems were investigated. For the initial urease activity studies, a simple microcell was used. Also, a test plate, containing dry reagent strips with all necessary chemicals was developed, making the analytical procedure very simple to perform. The test volume applied was 2 μl and the sensitivity to standards of mercury(II) ions is at least 0.005 μM (1.0 ng/ml). One sample could be analyzed in less than 8 minutes. Furthermore, the kinetics of sensor response versus enzyme activity is discussed.