The volume change for the E-Z photoisomerization reaction of the 3,3’-diethyloxadicarbocyanine iodide (DODCI) at 562 nm, and the 3,3’-diethyloxacarbocyanine iodide (DOCI) at 460 nm were determined in ethanol-water mixtures. The methods used were the solvent- and temperature dependent laser-induced optoacoustic spectroscopy (LIOAS) with nanosecond excitation and ca. 1-mu s heat-integration time. Contractions of 52 and 29 mL/mol, respectively, were measured for the DODCI photoisomerization. No photoisomerization volume change was observed for DOCI. The parameters obtained from calculations of the known initial and possible final conformations led to the conclusion that the volume change observed is mainly due to the rearrangement of solvent molecules around the DODCI isomers with different dipole moments.