Ligand photolysis and subsequent recombination in cytochromes b5 and c have been studied with picosecond resolution. In both proteins, an iron-histidine bond is broken after excitation with 314-nm light, and recombination occurs with a rate constant of about 1.4 x 10(11) s-1. Photolysis and reformation of the iron-histidine bond may be surprising as these hemoproteins do not reversibly bind ligands in nature. The findings are explained using results both from experiments on model hemes and from computer investigations with atomic resolution on the three-dimensional structure of the protein. After photolysis, the formation and recombination of the geminate contact pair are attributed to simple low amplitude ligand bond rotations, a result that can be applied to geminate processes in other hemoproteins and model heme compounds as well.