We present a novel approach for characterising the growth of aerosol droplets during the accommodation of a species from the gas phase. In particular, we illustrate that comparative growth measurements can be made on two aerosol droplets simultaneously, allowing the properties of different aerosols to be compared directly. A control droplet can provide a probe for monitoring the local gas phase composition with a fast time-response, while the variation in size and composition of the second droplet of interest can be investigated. We suggest that this strategy will allow accurate measurements of the thermodynamic and kinetic properties of aerosol droplets. (c) 2006 Elsevier B.V. All rights reserved.