Here, we present synergistically combined experimental and theoretical approaches to describe the molecular volume change associated with the photoisomerization of aqueous trans-azobenzene to cis-azobenzene. Although the cis isomer is sterically larger a volume contraction (trans-cis) of 4 mL/mol is observed by photoacoustic calorimetry in aqueous solution. Theoretical methods predict the same volume contraction and have determined the origin to be due to electrostriction arising from the newly formed dipolar species. (c) 2005 Elsevier B.V. All rights reserved.