This paper presents an experimental study on the effect of soiling on CPV modules located in the south of Spain. The I-V curves of two identical modules with different cleaning schedules (i.e. one was naturally soiled and the other one was cleaned periodically) have been measured over a course of year. The effect of soiling on the short-circuit current, maximum power and fill factor are presented together with the rain accumulation and particulate matter concentrations. Results showed that no losses occurred during the rainy periods, whereas for dry periods, a significant decrease in performance was observed due to soiling. Soiling was also found to have a greater influence on the maximum power rather than the short-circuit current. The highest power drop of up to 19% was measured after two dry summer months. Overall, the annual energy loss due to soiling was found to be 5.7%.