The influence of ionic strength (1-6 mol l(-1) NaClO4) on the parameters of adsorption isotherm(s) was studied using Cd2+-I-system as a model. Results, based on differential pulse polarographic measurements, simulation of polarograms and signal fitting procedure, indicate that the maximum surface concentration = (Gamma(max) = 1 x 10(-10) to (2.2 +/- 0.2) x 10(-10) mol cm(-2)), adsorption constant (beta = 3.1 x 10(5) to (1.55 +/- 0.35) x 10(7) 1 mol(-1)) and Frumkin coefficient (a = 0 to (0.5 +/- 0.1)) all increase with increasing electrolyte concentration. According to the model calculations, parallel increase of the adsorption constant and Frumkin coefficient should have the opposite influences on the shape of the reduction signal. In the system of interest, however, the former seems to be more important than the latter. As a result, gradual transformation of a single symmetrical peak to two close peaks can be observed on real polarograms at increasing electrolyte concentration.