Hexylferrocene phosphonic acid was adsorbed on indium-tin oxide (ITO) and amino-primed ITO electrodes from ethanol. The obtained monolayers are stable in acetonitrile where cyclic voltammetry shows the formation of domains of self-interacting and free ferrocene molecules. Distinct phases are observed on bare ITO, whereas a single Frumkin isotherm is obtained on amino-primed ITO. Dilution experiments with alkylphosphonic acids CnH2n+1PO(OH)(2) (n = 3, 6, and 12) have shown that only the dodecyl chain may compete with the hexylferrocene chain, which remains 50 times more strongly bound to the surface. The free energy of self-interaction of the ferrocene moieties in the layers is ca. 40 kJ mol(-1); that is, it is equivalent to that of a 15 carbon atom alkyl chain. Lateral interactions among hexylferrocene and dodecyl chains form isolated domains of the components, ruling out a real dilution of ferrocenes to single isolated molecules.