Interactions between the effects of soil mineralogy and of slope gradient on seal formation, infiltration rate (IR), runoff, and soil loss were evaluated, and the slope factor (Sf) functions for various soils with differing mineralogy were examined. Three different soils, on slopes of 9, 15, 20, and 25%, were subjected to 80 mm of simulated rainfall. The final IR was ≥20.5 mm h-1 in a clayey kaolinitic soil and ≤15 mm h-1 in smectitic soils. The total runoff, as a percentage of rainfall, ranged from 24 to 18%, 65 to 43%, and 50 to 35% from the kaolinitic, clayey smectitic, and sandy loam smectitic soils, respectively. The total soil loss ranged from 0.32 to 0.45 and from 1.14 to 3.93 kg m-2 for the kaolinitic and smectitic soils, respectively. In all three soils, increasing the slope gradient increased the detachment of coarse particles (>0.1 mm) more sharply than that of small particles (<0.1 mm), especially in the smectitic soils. Combining these results with previous findings for six soils indicated that soils could be divided into two groups according to their Sf values: (1) soils that contained smectite and were therefore dispersive and (2) soils that did not contain smectite and were therefore stable. For the former soils the regression Sf = 0.47exp(7.7 sin $theta$) defined the Sf values significantly, whereas for the latter group a linear regression Sf = 0.81 + 1.77 (sin $theta$) was required. It was suggested that the difference in the Sf functions was mainly due to seal formation enhancement by the smectite.