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A better knowledge of soil erosion by water is essential for planning effective soil and water conservation practices in semi-arid Mediterranean environments. The special climatic and hydrological characteristics of these areas, however, make accurate soil. loss predictions difficult, particularly in the absence of minimal data. Two zero-order experimental microcatchments (328-759 m(2)), representative of an extensive semi-arid watershed with a high potential erosion risk in the south-east of Spain, were selected and monitored for 3 years (1991-93) in order to provide information on the hydrological and erosional response. A pluviogram and hydrograph recorded data at 1-min intervals during each storm, after which the soil loss was collected and the particle size of the sediment was analysed. Runoff coefficients of about 9% and soil losses of between 84.83 and 298.9 g m(-2) year(-1) were observed in the area. Rapid response times (geometric mean values lower than 2 h) and low runoff thresholds (mean values between 3.5 to 5.9 mm) were the norm in the experimental areas. A rain intensity of over 15 mm h(-1) was considered as ’erosive rainfall’ in these areas because of the total soil loss and the transport capacity of the overland flow. Differences in pore-size distribution explained the different hydrological responses observed between areas. The erosional response was more complex and basically seemed to be determined by soil aggregate stability and topographical properties. A greater proportion of finer particles in the eroded material than in the soil matrix indicated selective erosion and the transport of finer material. Copyright (C) 2001 John Wiley & Sons, Ltd.