The effect of soiling in flat PV modules has been already studied, causing a reduction of the electrical output of 4% on average. For CPV's. as far as soiling produces light scattering at the optical collector surface, the scattered rays should be definitively lost because they cannot be focused onto the receivers again. While the theoretical study becomes difficult because soiling is variable at different sites, it becomes easier to begin the monitoring of the real field performance of concentrators and then raise the following question: how much does the soiling affect to PV concentrators in comparison with flat panels? The answers allow to predict the PV concentrator electrical performance and to establish a pattern of cleaning frequency. Sonic experiments have been conducted at the IES-UPM and CSES-ANU sites, consisting in linear reflective concentration systems, a point focus refractive concentrator and a flat module. All the systems have been measured when soiled and then after cleaning, achieving different increases of I(SC). In general, results show that CPV systems are more sensitive to soiling than flat panels, accumulating losses in I(SC) of about 14% on average in three different tests conducted at IES-UPM and CSES-ANU test sites in Madrid (Spain) and Canberra (Australia) Sonic concentrators can reach losses up to 26% when the system is soiled for 4 months of exposure. (C) 2010 Elsevier Ltd All rights reserved