The short-term and long-term capacity-time dependence of xanthine in 0.2 M NaCl at 35 degreesC inside the potential region of the capacity pit was measured at the mercury \ electrolyte interface. The short-term capacity-time dependence can be described well theoretically by 2D instantaneous nucleation with a constant growth rate and, for potentials near the more negative pit edge, with 2D nucleation according to an exponential law with a constant growth rate. The long-term capacity-time dependence corresponding to secondary adsorption can be modelled well by 1D instantaneous nucleation with a diffusion controlled growth rate. This mechanism refers to the formation of linear surface molecular stacks. The potential dependence of the overall nucleation and growth rate constant for the short-term and long-term capacity-time dependences will be discussed on the basis of the potential dependence of the interfacial supersaturation. (C) 2003 Elsevier Science B.V. All rights reserved.