In Chinese megacities the problems of air pollution are frequently obvious as hazy layers covering large areas combined with low visibility. To estimate the aerosol effect on regional and global climate, the knowledge of light-absorbing and light-scattering compounds as well as their mixing state is essential. A Volatility Tandem DMA (VTDMA) was used to measure nonvolatile fractions (at 300°C) of submicrometer aerosol particles. The remaining size distributions were divided into three fractions, corresponding to particles with a low-volatile, medium-volatile, and high-volatile fraction. The particles with a low-volatile fraction are equivalent with externally mixed nonvolatile (refractory) material; in the observed size range, this fraction is assumed to consist mainly of soot. Combined with number size distributions, the number and volume concentrations of externally and internally mixed nonvolatile particles were calculated. In a parallel study by Cheng et al. (2009) the results were used in a Mie model and compared with measured absorption coefficients showing a good agreement. During Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006), large variations in the mixing state were found, especially between new particle formation and heavily polluted days. The fraction of externally mixed soot decreased from ∼37% during clean to 18% during heavily polluted periods, parallel to an increase in particle mass concentration mainly caused by the production of secondary aerosol material. In the nucleation mode, high particle number fractions with high-volatile fractions were found (∼64% in contrast to ∼24% on a nonevent day); that is, refractory material is produced during nucleation and growth.