The interaction of aerosol particles composed of the protein bovine serum albumin (BSA) and the inorganic salts sodium chloride and ammonium nitrate with water vapor has been investigated by hygroscopicity tandem differential mobility analyzer (H-TDMA) experiments complemented by transmission electron microscopy (TEM) and K&#246;hler theory calculations (100-300nm particle size range, 298K, 960hPa). BSA was chosen as a well-defined model substance for proteins and other macromolecular compounds, which constitute a large fraction of the water-soluble organic component of air particulate matter. <P style="line-height: 20px;"> Pure BSA particles exhibited deliquescence and efflorescence transitions at <IMG WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0" src="acp-4-323-img1.gif" ALT="$sim$">35% relative humidity (<IMG WIDTH="30" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" src="acp-4-323-img2.gif" ALT="$RH$">) and a hygroscopic diameter increase by up to <IMG WIDTH="15" HEIGHT="15" ALIGN="BOTTOM" BORDER="0" src="acp-4-323-img1.gif" ALT="$sim$">10% at 95% <IMG WIDTH="30" HEIGHT="14" ALIGN="BOTTOM" BORDER="0" src="acp-4-323-img2.gif" ALT="$RH$"> in good agreement with model calculations based on a simple parameterisation of the osmotic coefficient. Pure NaCl particles were converted from near-cubic to near-spherical shape upon interaction with water vapor at relative humidities below the deliquescence threshold (partial surface dissolution and recrystallisation), and the diameters of pure NH₄NO₃ particles decreased by up to 10% due to chemical decomposition and evaporation. <P style="line-height: 20px;"> Mixed NaCl-BSA and NH₄NO₃-BSA particles interacting with water vapor exhibited mobility equivalent diameter reductions of up to 20%, depending on particle generation, conditioning, size, and chemical composition (BSA dry mass fraction 10-90%). These observations can be explained by formation of porous agglomerates (envelope void fractions up to 50%) due to ion-protein interactions and electric charge effects on the one hand, and by compaction of the agglomerate structure due to capillary condensation effects on the other. The size of NH₄NO₃-BSA particles was apparently also influenced by volatilisation of NH₄NO₃, but not as much as for pure salt particles, i.e. the protein inhibited the decomposition of NH₄NO₃ or the evaporation of the decomposition products NH₃ and HNO₃. The efflorescence threshold of NaCl-BSA particles decreased with increasing BSA dry mass fraction, i.e. the protein inhibited the formation of salt crystals and enhanced the stability of supersaturated solution droplets. <P style="line-height: 20px;"> The H-TDMA and TEM results indicate that the protein was enriched at the surface of the mixed particles and formed an envelope, which inhibits the access of water vapor to the particle core and leads to kinetic limitations of hygroscopic growth, phase transitions, and microstructural rearrangement processes. <P style="line-height: 20px;"> The K&#246;hler theory calculations performed with different types of models demonstrate that the hygroscopic growth of particles composed of inorganic salts and proteins can be efficiently described with a simple volume additivity approach, provided that the correct dry solute mass equivalent diameter and composition are known. A parameterisation for the osmotic coefficient of macromolecular substances has been derived from an osmotic pressure virial equation. For its application only the density and molar mass of the substance have to be known or estimated, and it is fully compatible with traditional volume additivity models for salt mixtures.