Organic/inorganic complex is the most important form of substance in the soil. Aggregation kinetics and structural characteristics of yellow earth colloids, humic colloids and yellow earth/humic colloids were studied as affected by CaCl₂ concentration. Results indicate that the aggregation of yellow earth colloids, humic colloids and yellow earth/humic colloids was strongly dependent upon concentration of the electrolyte, that is, CaCl₂, with Critical Flocculation Concentration (CFC) being 1.38 mmol L^-1, 45 mmol L^-1 and 15 mmol L^-1, respectively. When CaCl₂ concentration was higher than the CFCs, aggregation of the colloids proceeded in fast diffusion-limited cluster-cluster aggregation (DLCA), thus forming loose aggregates with fractal dimension being 1.45, 1.46 and 1.76 for yellow earth colloids, humic colloids and yellow earth/humic colloids, respectively. However, when CaCl₂ concentration got lower than the CFCs, aggregation of the colloids became very sensitive to the concentration of the electrolyte and displayed as slow reaction-limited cluster-cluster aggregation (RLCA), thus leading to formation of relatively compact aggregates with fractal dimension being up to 1.58, 1.56 and 1.86, separately, for the three types of colloids. Thus, the faster the aggregation velocity was, the looser the structure of aggregates would be. The findings indicate that humus contributes significantly to stability of the structure of soil mineral aggregates.