Interrill runoff and sediment yield were measured from a bare and from mulch-covered soil surfaces, (1) to evaluate the effects of mulch size and mulch incorporation during a simulated high-intensity rainstorm and (2) to understand the mechanisms involved. Mulch elements used consisted of 2-cm-thick extruded polystyrene cut into squares with sides of 3.0, 5.9, 11.7 and 22.3 cm. Each of them was applied at four cover levels: 30, 49, 70 and 88%. A loamy sand soil, susceptible to surface sealing, was used in all experiments. The experimental results clearly indicate that for a given cover percentage, the effectiveness of different mulch sizes in reducing runoff and soil loss from interrill soil surfaces is inversely proportional to the size of the individual mulch elements. This finding is attributed to a positive effect of mulch size on mulch flow volume, and hence on the probability of runoff generation. Runoff and sediment yield were higher when mulch elements were partly incorporated into the top layer than when mulch elements were placed on the soil surface. The results of this study may be used as a guide for the selection of mulch type and for the application of mulches to soils susceptible to surface sealing.