Kinetics of ammonium fixation in several types of cultivated upland soils and paddy soils derived from represen tative parent materials in Hunan province, China, was studied through sampling in fields, and incubation and determination in laboratory. Results showed that fixation of added NH₄⁺ in the tested soils, whether paddy or upland, all increased with fixation time, and the fixing rate for added NH₄⁺ was high at the initial stage, especially at the initial 8~12 hours of reaction, and grad ually slowed down afterward. After 24 hours, the fixation of added NH₄⁺ reached equilibrium basically. Both the first order equation and Elovich equation were the most applicable to describing NH₄⁺-fixation kinetics in the tested soils. The parabolic dif fusion equation had some good fitting results, too. But the zero level equation was not applicable. The kinetic parameters obtained through the first-order equation, i. e., theoretical maximum fixation capacity (A), reaction rate constant (b) and half reaction time, varied with soil types. The paddy soils were lower in theoretical maximum fixation capacity (A) and half reaction time, but much higher in reaction rate constant (b) than the upland soils.