A light scattering optical particle counter (OPC) was re-calibrated for a well-defined particle material, to convert optical equivalent size both into aerodynamic and volume equivalent diameters. For specific test dusts, the calibrated OPC provides a direct read-out dust mass vs. aerodynamic diameter and - by weighting the appropriate size bins with the standard transmission curve - as total PM2.5 mass. The time resolution is in the order of 1 s. The paper gives details of an accurate calibration method for supermicron particle materials where monodisperse size standards are generally unavailable. The procedure is based on measuring the transmission curve of an aerodynamic collection device in parallel with the optical particle counter and an aerodynamic particle sizer. The calibration was verified in more than 100 tests, where particle emissions from bag-house filter media were recorded in parallel with the recalibrated OPC and a PM2.5 cyclone (BGI SCC). The deviation between OPC and SCC is less than 20% for about 95% of the results, with no systematic deviation. (c) 2007 Elsevier Ltd. All rights reserved.