A prototype real-time mass-weighted aerodynamic particle sizer has been developed to provide on-line, real-time measurements. The aerosol is aerodynamically classified by means of inertial separation. However, in contrast to most cascade impactor techniques, the ratio of the collection substrate area to wall area is high, thus reducing wall loss effects. The collection substrate is mechanically vibrated at its harmonic frequency. Providing that this frequency is undamped, it can be related unambiguously to the total mass of the oscillator. Since this mass comprises, the substrate, the containing fluid and the deposited particulate matter, the concentration of the aerodynamically classified deposit can be determined.Mass sensitivities of approximately 10 - 8 g have been obtained, using both single- and multiple-stage devices, which result in a near-real-time performance lower limit of 1 μgm - 3 . Stage mass loading limits of 10 mg have been achieved on uncoated substrates without significant re-location, particle bounce or re-entrainment problems being encountered. Each stage can be isolated and cleaned to provide both maintenance and time-average physiochemical data.