Volume changes associated with intermolecular electron transfer reactions between (Ru(bpy)(3)(2+) and Fe(H2O)(6)(3+) and between xanthene dyes and metal cyanides in aqueous solutions were measured by temperature-dependent laser-induced optoacoustic spectroscopy with nanosecond pulsed excitation. For electron transfer between Ru(bpy)(3)(2+) and Fe(H2O)(6)(3+) a contraction of 11 mL per absorbed Einstein was observed. Smaller volume changes (less than or equal to 5 mL/Einstein) were determined for the electron transfer between xanthene dyes and metal cyanides. A contraction resulted for the photooxidations while an expansion was observed for the photoreductions. In each case the volume change is largely attributed to the medium reorganization around the system present after the time window of the experiment (ca. 600 ns) rather than to intrinsic molecular parameters, such as, e.g., bond length changes.