Therefore, the inverted distribution alone cannot be the reason for the improved activity, even though it could be a contributing factor -and the same applied to the seemingly longer M-O bonds in inverse configurations, which would entail more labile lattice oxygens, therefore more prone to reaction. Our results point to a boost connected to specific features of the reactivity of Mn such as, for example, its large number of possible oxidation states. The surface enrichment in Mn, as estimated by XPS, could have amplified this effect. Much like NiCo2O4, in ZnCo2O4 the guest metal can arguably be present only as a Zn 2+ cation. This implies that Co is present only in the form of Co 3+ , supposedly the active species; moreover, ZnCo2O4 is a normal spinel, and it could be argued that ZnCo2O4 is a direct analogue of Co3O4 whose Co 2+ cations were substituted by Zn 2+ . In this view, the catalytic results suggest that the low activity of ZnCo2O4 is either due to an absence of a synergetic effect of Co 2+ , or an inhibiting effect by Zn 2+ , promoted by the enrichment in Zn on the surface. Conversely, Fe prefers the trivalent Fe 3+ form, thus all Co in CoFe2O4 should be Co 2+ .