We used X-ray photoelectron spectroscopy (XPS) to study the elemental composition and the metal oxidation states of the upper 15-20 nm of both MAs before and after 7 h of electrolysis (Figure , S4F, and S13). For MA1, two distinctive peaks attributed to the 2p orbitals of Fe 2+ were observed at 708.6 eV (2p3/2) and 721.2 eV (2p1/2). The binding energies did not change and no peaks are present that would indicate the formation of iron oxides. The overall oxidation state (2+) of the palladium is also not affected, but the peaks of the 3d orbitals are slighly shifted from 337.4 eV (3d5/2) and 342.6 eV (3d3/2) to 338.4 eV (3d5/2) and 343.6 eV (3d3/2), respectively (Figure ). These shifts may indicate an exchange of Cl -with ClO4 -, which is present in excess in the electrolyte solution (Figure ). The amount of Cl -is significantly reduced and ClO4 -is observed. Changes in the oxidation state of the palladium centers during the reaction most likely facilitates this anion exchange; XPS analysis of MA2 reveals a similar effect (Figure ). The changes are also consistent with the formation of palladium hydroxides. There is a correlation between the catalytic performance and the number of deposited layers, as well as the similarity between MA1 and MA2, as determined from CV measurements. For both assemblies, the catalytic performance increases up to 10 layers, and subsequently levels off, upon increasing the thicknesses of the assemblies. This observation suggest that not all palladium centers are catalytically active for the MAs prepared by 15 deposition cycles, the electrochemical water splitting might occur only at the top layers. The EC experiments with MA1 indicate that all iron complexes are electrochemically addressable. It is know that small redox-probes in solution can be electrochemically addressed by electrodes surfaces coated by assemblies similar to the MAs used in this study. The similarity between the catalytic activities of MA1 and MA2 suggest the formation of similar amounts of the same catalysts.