We explored the conditions of the covalent grafting of organic-inorganic polyoxometalate hybrids onto gold using two basic reactions: the peptide-type coupling between a functionalized POM and a preformed organic self-assembled monolayer and the direct attachment of a thiol-terminated POM. We aimed at demonstrating that the variability of the POM loading is an inherent parameter of chemical reactions carried out under heterogeneous conditions that should be systematically assessed. Mitigation measures include a careful preparation and characterization of the substrates at every step before the final POM anchoring. Moreover, a very fast electron transfer across a gold microelectrode and a POM layer was achieved in the case of a very short tether.