This article reports on methabenzthiazuron [1-(1,3-benzothiazol-2-yl)- 1,3-dimethylurea] (MBT) adsorption process on six agricultural allophanic and nonallophanic soils. The effect of amendment with exogenous organic matter was also studied. Adsorption kinetic fits an hyperbolic model. MBT adsorption reached an apparent equilibrium within 2 h and followed a second-order reaction. The maximum adsorbed amounts for natural soils ranged from 32 to 145 μg g⁻¹. Rate constants were considered relatively low (0.27 − 1.5 × 10⁻⁴ [μg g⁻¹]^1-n s⁻¹); the slow process was attributed to a combined effect of difussion and adsorption. MBT adsorption fits the Freundlich model with r values ≥ 0.998 at P ≤ 0.001 significance levels. K _f and Freundlich exponents (1/n) ranged from 5.3 to 82.1 cm³ g⁻¹ and from 0.66 to 0.73, respectively. K _f values for soils with a low organic matter content were lower than that obtained from the only typical allophanic soil derived from volcanic ash under study. Lineal regression analysis between K _f and organic matter content of nonallophanic soils gave a correlation coefficient of 0.980 (P = 0.02). Dispersion of K _d values together with close values of K _OM indicate that organic matter (OM) was the principal component responsible for MBT adsorption in unamended soils. Addition of peat decreased soil pH and increased adsorption capacity for allophanic and nonallophanic soils. Kinetic experiments showed enhancements of X _max values and lower rate constants.