In previous report (Ceh, B.; Lasic, D. D. Langmuir 1995, 11, 3356), the accumulation of external molecules into preformed liposomes by the influence of various gradients, a very important feature in recent applications of liposomes as drug carriers, was presented. The thermodynamic model of liposome loading with weak bases/acids used preset permeability coefficients that allowed or disallowed penetration of noncharged and charged species, respectively, across the vesicle membrane. Now in this article we investigate the kinetics of drug loading. Using Fick＇s first law, we can calculate kinetics of loading, expressing the diffusion constant as a permeability coefficient and defining an associated now rate. Results of this theoretical approach, which in the limit of long times converge to the result of the thermodynamic model, show the filling of liposomes with particular molecules as a function of time, as well as time dependence on pH changes and concentration of any other species in the liposome internal or external environment.