We therefore suggest that the seeming incompatibility between the XFEL and EPR data for the S 3 state, lies in the fact that the oxo-hydroxo and [O 2 ] 3-are in equilibrium. The [O 2 ] 3- form detected in the XFEL determined structure is not readily apparent in the EPR spectrum due to it S=6 nature and resultant low intensity compared with the S=3 form. Further simulations presented in the Figure suggest that the S=6 component is also likely a major component of the broadened W-band EPR spectrum caused by methanol and glycerol addition. It has been known for some time that the S=3 species does not correspond to all of the S 3 spin and that an EPR "undetectable" component observed only on near infrared (NIR) irradiation is also present in equilibrium with it. In our analysis, this undetectable component corresponds to the [O 2 ] 3-form. This is a different assignment to that previously made for the S=6 form detected in spinach samples where the S=6 form was attributed to a closed cubane form of the WOC cluster with a pentacoordinated Mn 4 (IV) ion, an intermediate formed prior to binding of the second substrate water. This however, in striking contrast to the model proposed here, is not supported by the XFEL structural data. In addition it has been shown that Mn(III) is required for NIR excitation and the large D value of 1.523 cm -1 for the S=6 form strongly suggests the presence of Mn(III) ion in the complex. It should be noted that it is possible that the peroxo form, Figure , is also present in a low concentration and its EPR spectrum is masked by the oxo-hydroxo form. The peroxo complex would have two Mn(III) ions present likely leading to a large D value similar to the [O 2 ] 3-form which would again lead to a low intensity EPR spectrum compared with the oxohydroxo form.