The electrochemical impedance method has been used for the quantitative study of uracil adsorption kinetics at the bismuth single crystal plane aqueous Na2SO4 solution interface. Analysis of impedance data demonstrates that the adsorption process of uracil is complicated and limited by the mixed kinetics, i.e. by the slow adsorption and diffusion steps. Analysis of the Cole-Cole and other dependences indicates that, at small frequencies and higher uracil concentrations, two-dimensional association of the uracil molecules is possible. Nonlinear regression analysis has been used for fitting the experimental complex plane (Z", Z') plots. The classical Frumkin-Melik-Gaikazyan circuit in the region of higher ac frequencies and lower uracil concentrations (c(uracil) less than or equal to 2 x 10(-3) M), and the modified Frumkin-Melik-Gaikazyan equivalent circuit (taking into account the inhomogeneous semi-infinite diffusion) at c(uracil) greater than or equal to 2 x 10(-3) M fit with a reasonable accuracy the experimental Z", Z'-plots. It was found that the values of the double layer capacitance, partial charge transfer resistance, adsorption capacitance and Warburg-like diffusion impedance depend noticeably on the electrode potential and concentration of uracil. Other more complicated equivalent circuits have been used for fitting the experimental impedance data. (C) 2003 Elsevier B.V. All rights reserved.