Vehicle exhaust aerosol characterisation requires real-time instruments to record particle concentration over transient tests. This work demonstrates the application of a real-time prototype diffusion charger to characterise exhaust aerosol. The output signal of the diffusion charger is first cross-compared with different particle properties and is shown to closely correlate with the active surface. The instrument is then calibrated using monodisperse diesel exhaust particles. The calibrated signal is shown to precisely estimate the particle surface area obtained from integration of different size distributions, including nucleation mode particles, in a range of more than five orders of magnitude. The mean particle diameter is also estimated in real time, combining the diffusion charger signal with aerosol number concentration provided by a particle counter. Finally, we demonstrate the potential to use the active surface for establishing vehicle exhaust particle emission levels, similarly to the regulated gravimetric procedure applied today. (C) 2004 Elsevier Ltd. All rights reserved.