Abstract— Employing time-resolved photoacoustics we measured the structural volume changes (ΔVri) occurring after photo-excitation of blepharismin (BPR) aqueous solutions; an expansion occurring in the subnanosecond timescale is followed by a back contraction within some hundreds of nanoseconds. The magnitude of the ΔVri strongly depends on pH, allowing the determination of the pKa of BPR with this method. The values so measured are very close to those found by means of UV-visible absorption spectroscopy. The presence of water-soluble electron acceptors or donors (hexacyanoferrate [III] hexacyanofer-rate [II]) as well as the concentration of oxygen do not affect the magnitude or the kinetics of the structural volume changes. On the contrary, we detect a strong deuterium effect; this suggests that the observed ΔVri are related to an altered hydrogen bond pattern of the excited state of the pigment with respect to the ground state. Comparative measurements with the parent compound hypericin are also reported, suggesting that the photo-induced expansion-contraction pattern is a general characteristic of polyhydroxylated quinones.