Laser-induced optoacoustic spectroscopy (LIOAS) was applied to the study of the photolysis of the CO-ligated heme protein horseradish peroxidase isoenzyme C (HRP). Laser photolysis produced structural volume changes faster than 50 ns. The photoreaction volume and enthalpy changes were determined by means of temperature-dependent measurements in the range 6-23 degreesC. The volume change (+29.6 mL/mol) can be mainly attributed to the displacement of CO to the bulk solvent. The enthalpy change is mainly related to the Fe-C bond energy with little contribution from the protein matrix. The results are interpreted in terms of the structural properties of HRP, which has a direct exit channel from the heme to the solvent, and compared to related studies on the CO complexes with myoglobin and hemoglobin.