Photoacoustic calorimetry and femtosecond transient absorption spectroscopic techniques have been used to study the electronic relaxation and ligand binding reactions that occur following photoexcitation of the 345 nm peroxide-to-copper ligand-to-metal charge transfer (LMCT) band of oxy hemocyanin (Hc) from Limulus polyphemus and Busycon canaliculatum. The results indicate that the LMCT state in both proteins decays to the ground state via a nonradiative mechanism with a time constant of similar to 610 fs. The short lifetime of the LMCT state measured in this work provides an explanation for why the oxy He excited state exhibits a low fluorescence yield. The results also suggest that photoexcitation of oxy He leads to the dissociation of O-2 (by a thermal or photochemical process) and that the O-2 molecule subsequently recombines with the metal center on the 80 ps time scale. The time-resolved data reveal that the dynamics in the vicinity of the active site are identical for the two types of Hcs studied, which is consistent with the similarity of the active sites in various Hcs.