The effect of dust on the performance of photovoltaic concentrators has been investigated. The dust concentration in the air was measured continuously during the test period, and the rate of dust accumulation on the concentrator's surface was determined. For the purpose of comparison, an identical concentrator, the surface of which was kept clean, was evaluated simultaneously with the dusty concentrator. The change of theI/Vcharacteristics as a result of dust accumulation was related to the amount of dust accumulated per unit area of the collector surface (g/m2). It has been shown that the major reductions in the short-circuit current, cell temperature and the efficiency occur as the dust starts to deposit onto the collector surface, but the rate of decrease is slower for dust accumulations beyond 2 g/m2. It is argued that the dust accumulation per unit area is a more representative parameter than the exposure time for such studies.