Adsorption of well-known surfactant Triton-X-100 (T-X- 100) and mixed systems comprising T-X- 100 with o- or p-nitrophenots, at the mercury/electrolyte solution interface versus T-X- 100 bulk concentration has been studied. Diversified approach comprising capacitive current measurements, desorption peak height analysis, fractal analysis, nonthermodynamic calculation of coefficient of lateral interaction, calculation of relative coverage assuming flat and perpendicular dispositions of molecules, fit with Flory-Huggins isotherms corresponding to different molecular orientations, and predictions of random sequential adsorption (RSA) and equilibrium (generalized RSA) theories, together with analysis of reduction mechanisms of nitrophenols has been used. It is found that the adsorbed layer of T-X-100 and mixed layers are fractal. Fine structural changes of adsorbed layer(s) are identified and related to corresponding changes in fractal dimension. Evidence derived from alternating current (ac) and square wave (sw) voltammetric measurements and theoretical considerations support these conclusions. For the adsorption of T-X-100 four different adsorption phases, corresponding to increasing bulk concentration, are identified and characterized. Mixed systems undergo different changes in microstructure that are reflected in observed changes of reduction mechanisms of nitrophenols. Those are in turn related to the differences in molecular interactions between T-X-100 and certain nitrophenol. The results are relevant for study of any system exhibiting fractal features and accessible to electrochemical methods such as adsorbed films, models of biological membranes, etc. (C) 2004 Elsevier Ltd. All rights reserved.