Cytochrome P450 (P450) and chloroperoxidase (CPO) are thiolate-ligated haem proteins that catalyse the activation of carbon hydrogen bonds. The principal intermediate in these reactions is a ferryl radical species called compound I. P450 compound I (P450-I) is significantly more reactive than CPO-I, which only cleaves activated C-H bonds. To provide insight into the differing reactivities of these intermediates, we examined CPO-I and P450-I using variable-temperature Mössbauer and X-ray absorption spectroscopies. These measurements indicate that the Fe-S bond is significantly shorter in P450-I than in CPO-I. This difference in Fe-S bond lengths can be understood in terms of variations in the hydrogen-bonding patterns within the 'cys-pocket' (a portion of the proximal helix that encircles the thiolate ligand). Weaker hydrogen bonding in P450-I results in a shorter Fe-S bond, which enables greater electron donation from the axial thiolate ligand. This observation may in part explain P450's greater propensity for C-H bond activation.