The behaviour of self-assembled monolayers of thiohexadecanoic acid adsorbed onto gold interacting in assymmetric 2:1 electrolytes has been studied with direct force measurements. The effects of two divalent cations (Mg2+ and Ca2+) were studied at concentrations ranging from 1 µM to 10 mM. As compared to interactions in the presence of Na+, the divalent ions adsorb strongly to the surfaces, with the effect of lowering the surface potential, and decreasing the double-layer repulsion. At concentrations above 10 µM, the Ca2+ ions were found to adsorb stronger than Mg2+, causing charge reversal at higher concentrations, and the net interactions at 10 mM were attractive over the measurable range.