A new PM2.5 inlet, based on the particle cup impactor configuration, was designed for sampling fine particles smaller than 2.5 mum in aerodynamic diameter and for operating at a flow rate of 5 l/min, as the devices, which are used to analyze the chemical composition of the particles, have good efficiency only at low-volume flow rates. The performance of the inlet was evaluated in a test chamber, and the optimum dimensions of the particle cup impactor were determined by varying the nozzle-to-cup distance. Additional experiments covering flow rates between 3 and 10 l/min with particle sizes between 0.8 and 5.0 mum were carried out in the test chamber. The performance indicated that a nozzle-to-cup distance of 1.1 mm would yield a sharp size cutoff. The results from the tests showed that the inlet had a cutoff size of 2.55 mum in aerodynamic diameter at a flow rate of 5 l/min.