With the combination of a hygroscopicity tandem differential mobility analyzer (HTDMA) and an optical particle counter the relation between hygroscopic and optical properties has been investigated for 300 nm urban atmospheric particles. Systematic new features of the state of mixture of optically important submicrometer particles were revealed. Besides a high fraction of absorbing (soot) chains, the most hydrophobic 300 nm particles had a high (≈30%) number fraction that is classified as transparent spheres, which could be anything from glassy spheres to high molecular hydrophobic organic droplets. These optical modes need to be considered when using HTDMA growth factors to convert dry aerosol properties to climatically relevant ambient levels. ¿ 2001 American Geophysical Union