Myoglobin, a small globular haem protein that binds gaseous ligands such as O-2, CO and NO reversibly at the haem iron, serves as a model for studying structural and dynamic aspects of protein reactions. Time-resolved spectroscopic measurements after photodissociation of the ligand revealed a complex ligand-binding reaction with multiple kinetic intermediates, resulting from protein relaxation and movements of the ligand within the protein(1-3). To observe the structural changes induced by ligand dissociation, we have carried out X-ray crystallographic investigations of carbon monoxy-myoglobin (MbCO mutant L29W) crystals illuminated below and above 180 K, complemented by time-resolved infrared spectroscopy of CO rebinding. Here we show that below 180 K photodissociated ligands migrate to specific sites within an internal cavity-the distal haem pocket-of an essentially immobilized, frozen protein, from where they subsequently rebind by thermally activated barrier crossing. Upon photodissociation above 180 K. Ligands escape from the distal pocket, aided by protein fluctuations that transiently open exit channels. We recover most of the ligands in a cavity on the opposite side of the haem group.