Previous studies have shown that runoff generation on conventionally tilled fields on loamy soils is primarily controlled by soil surface characteristics. In this study we investigate the factors controlling runoff generation and interrill erosion on fields under reduced and no-tillage in central Belgium. Over a period of 5 years, 184 rainfall simulation experiments on small-scale plots (∼0.73 m2) were conducted on fields planted with maize and sugar beet. The results clearly confirmed the beneficial effects of reduced tillage in terms of runoff and soil loss, with a reduction in 63 and 88% of the cases, respectively, but they also indicated that the effect of the application of conservation tillage was highly variable. The erosion-reducing effect of conservation tillage was more pronounced when large absolute soil loss rates on the conventionally-tilled part were recorded. Empirical regression models are proposed to explain observed variations. Overall, factors relating to the surface state of the soil, such as crusting and total cover, appear to be much more important in controlling runoff than intrinsic soil characteristics, such as organic matter content and texture. Thus, soil surface characteristics control runoff generation on loamy soils, also when conservation tillage is applied. Whereas a single empirical model for the description of the runoff response could be used for all the tillage techniques, separate relationships for each tillage technique were required for soil loss.