Adsorption isotherms of the insecticidal protein of Bacillus thuringiensis subsp. kurstaki (Btk) in red soil and brown soil were determined using the equilibrium adsorption method. Based on the isothems, their adsorption equilibrium constants and thermodynamic parameters were calculated. Results show that in the range between 278K and 318K, the Btk protein adsorption isotherms in red soil and brown soil followed the Langmuir equation (R²﹥0.994 1), showing that Bt adsorption and adsorption equilibrium constant decreased with rising temperature, but separation factor constant (R_L) increased in the range of 0.218 1～0.580 1 in red soil and of 0.361 7～0.754 1 in brown soil, indicating that both were of preferential adsorption. Btk protein adsorption in the two soils was a spontaneous, exothermic, entropy-increasing process. In red soil the activation energy of adsorbed Btk protein ranged from 13.43～14.78 kJ mol^-1, while in brown soil it did from 10.89～11.47 kJ mol^-1. Under rising temperature, it increased along with adsorption free energy, which suggests that the adsorption of Btk protein by the soils is turning from a physical process into a chemical one.