Further experimental support for our S 3 state model comes from analysis of the X-ray emission spectroscopy (XES) data by Ibrahim et al. The 1F flash first moment XES shift can be confidently assigned to the Mn (III) to Mn (IV) oxidation of Mn 4 . The first moment shift for the 2F state is approximately 40% of the 1F shift based on the solution phase data and the most current time resolved crystal data, see Figure . In addition at least 10% of the oxidation change shift can be attributed to S 1 to S 2 oxidation based on the S state populations of the 1F state given in Abrahim et al. leaving around 30% Mn oxidation occurring in the S 2 to S 3 transition. This is what is predicted by our equilibrium model above. The 30% Mn oxidation can be attributed to formation of the oxo-hydroxo form where a Mn 1 (III) to Mn 1 (IV) oxidation occurs. The [O5O6] 3-form has however an identical overall Mn oxidation state to the S 2 state i.e. one Mn(III) and three Mn(IV) so will not give rise to a first moment shift.