A new system for simultaneous determination of individual particle size, hygroscopicity, and composition of internally and externally mixed aerosols is presented. An HTDMA is used to select particles according to their size and hygroscopicity for compositional analysis by a single particle mass spectrometer. The capability of the system is demonstrated on a laboratory generated aerosol consisting of polystyrene latex spheres coated with a layer of ammonium nitrate to represent an internally mixed aerosol with controllable hygroscopicity that is externally mixed with pure ammonium nitrate particles. Results are presented from an ambient study illustrating the sensitivity of the system and the capability to provide detailed information on individual atmospheric particles within a short sampling time.