The electrosorption behaviour of met himazole(vetranal) at the mercury/1 M NaClO4 interface was determined from a double layer differential capacity measurements extrapolated to zero frequency. Methimazole is the thiourea derivative, which is used as a drug in the overactivity of the thyroid gland. The values of zero charge potential were shifted to negative potentials ca. 230mV with an increasing methimazole concentration from 10(-4) M to 10(-1) M. The relative surface excesses Gamma' of methimazole were calculated from surface pressure at the density of surface charge sigma = constant. The values of Gamma' increase with the increase of or. Adsorption of methimazole was analyzed on the basis of the constants obtained from the Frumkin and virial isotherms. The values of the standard Gibbs energy of adsorption Delta G degrees increase with the increase of or while the values of interaction parameters A and B decrease. (c) 2005 Elsevier Ltd. All rights reserved.