MYOGLOBIN is a globular haem protein that reversibly binds lig-ands such as O₂ and CO. Single photons of visible light can break the covalent bond between CO and the haem iron in carbonmonoxy-myoglobin (MbCO) and thus form an unstable intermediate, Mb^*CO, with the CO inside the protein^1,2. The ensuing rebinding process has been extensively studied as a model for the interplay of dynamics, structure and function in protein reactions. We have used X-ray crystallography at liquid-helium temperatures to determine the structure of Mb^*CO to a resolution of 1.5 Å. The photodissociated CO lies on top of the haem pyrrole ring C. Comparison with the CO-bound and unligated myoglobin structures reveals that on photodissociation of the CO, the haem 'domes', the iron moves partially out of the haem plane, the iron–proximal histidine bond is compressed, the F helix is strained and the distal histidine swings towards the outside of the ligand-binding pocket.