Surface charge characteristics of the clay fraction of a laterite derived from basalt and a red earth derived from Quaternary red clay in southern China were investigated based nn a model for describing charge characteristics of soil systems containing both permanent and variable charage minerals. pH₀, ZPNC and IEP were used as indexes to characterize surface charge Properties. The pH₀, denoting zero point of charge of variable charge system, was determined with potentiometric titration method. ZPNC, which means zero point of net charge of mixed systems with variable and permanent charge, was treasured with ion adsorption method. IEP was determined by electrophoresis method. The pH₀ value, ZPNC,and lEP of the laterite were higher than that of the red earth due to differences in clay mineral composition and in content of oxides of iron and aluminum between these twa soils. Because the quantity of perrnarxent negative charge of the red earth was comparatively high, the difference between pHo and ZPNC for this soil was more remarkable. After the addition of 3% organic matter and incubation for four moxiths under submergic corAditxons, the humus content of the clay fraction of these soils increased by less than 0.5%, its effect on the pH₀ value was indistinct.