Kinetics of humic acids interactions with proton and calciumions were studied using modified potentiometric titration.Solutions of 1 and 2 g L^-1 humic acids in 0.1 mol L^-1 NaClO₄ or KCl were titrated with 0.1 mol L^-1 HClO₄ or HCl from pH 7.5 to about 2.6 at 24℃ under carefully controlled anaerobic conditions.The reaction kinetics were well described by a traditional relaxation equation Y=Y₀EXP(-t/S). In each titration point occurred an initial fast relaxation of redox potential(E_R)and glass electrode potential(E_G),followed by further slow relaxation and/or oscillations of E_R.And eventually,the reaction reached a steady state,indicating the non-equilibrium nature of humic acid solution system.The analyses of time-resolved data of E_G for the stepwise addition of calcium ions to humic acid solutions revealed that the overall reaction rates were diffusion-controlled rather than reaction-controlled.After relaxation down to a steady state E_R showed a linear pH dependence with an almost constant slope of 45.3? 1mV/pH unit despite the differences of humic acid concentrations and supporting electrolytes,which then could be applied as an index of the arrival at a steady state of the humic acid interactions with metations.The characteristics of proton reactions with humic acids were studied using the suggested index,and well reproducible results were obtained.Four types of binding sites HA,H₂A,H₃A and HA₂ with accordingly apparent pK values of 5.79~5.82,3.77~3.84,1.64~1.71 and 2.75~2.79 were found existing in the studied humic acids.