Ion diffusion is an important path of mass transport in nature，controlling movement of nutrients from bulk soil to plant roots as well as fate of potentially harmful salts and toxic compounds.The kinetic method was used in this experiment to determine surface charge properties of soil colloids of neutral purplish soil and latosol and to study K⁺，Mg²⁺ diffusion in the two types of colloids under the same condition as surface charge properties were determined.And the diffusion flux dependent of time and diffusion coefficient in the two types of colloids (at different of pH electrolyte type and electrolyte concentration) were determined. Results show 1) the total diffusion flux varied in the range of 2.6 ~3.1 mg g^-1 in neutral purple soil under different environmental conditions，about 6~10 times higher than what in latosol，being in the range of 0.26 ~0.51 mg g^-1; and 2) the diffusion coefficient varied in the range of 1.17×10^-9~1.91×10^-9 cm² s^-1 in neutral purple soil under different environmental conditions，and in the range of 0.23×10^-9~0.45×10^-9 cm² s^-1 in latosol. Surface charge properties were the dominant factor affecting ion diffusion in charged colloids.