A tandem differential mobility analyser system combining volatilisation and humidification aerosol conditioning to allow the observation of hygroscopic behaviour for non-volatile residues in sulphate aerosols in real time has been developed and demonstrated. The volatilisation and humidification tandem differential mobility analyser (VH-TDMA) system can measure changes in the hygroscopic behaviour of heterogeneously nucleated aerosol as one or more semi-volatile species are removed through thermal evaporation or decomposition. The VH-TDMA system has the potential to distinguish between submicrometer particles containing residues of differing volatility and hygroscopic growth and can do so for residue diameters as small as 5 nm. The technique has the potential to distinguish between components such as sea salt and combustion-derived particles within submicrometer sulphate aerosols and will find application in marine and combustion aerosol research investigating nucleation, coagulation and secondary aerosol fort-nation processes. Hygroscopic growth and volatility data measured with the system are presented for di-2-ethylhexyl-sebacate, sodium chloride, ammonium nitrate, ammonium sulphate, sulphuric acid, ammonium bisulphate and methane sulphonic acid, as single aerosol species for comparison with values reported by other authors. Results of investigations of aerosols containing mixtures of two species are also presented. (C) 2003 Elsevier Ltd. All rights reserved.