The binding of ferritin to Langmuir monolayers was monitored through enhancement in light reflection normal to the air/water interface. Single component monolayers of eicosylamine, and binary mixtures of dioctadecyldimethylammonium bromide (DOMA) and methyl stearate (SME) were used to test the influence of monolayer charge type and density on ferritin adsorption. The absence of ferritin binding to the neutral SME films and a diminished binding to the positively charged DOMA and eicosylamine films from high ionic strength subphases suggested an electrostatic based adsorption mechanism. Surprisingly, eicosylamine films bound more ferritin than DOMA films, even though DOMA films demonstrated a more positive surface potential. However, as DOMA was progressively diluted in SME, ferritin binding increased, reaching a maximum value, similar to that of eicosylamine, for an SME–DOMA=6:1 molar ratio. Although employed as a charge diluent, miscibility analysis indicated that SME actually increased the net surface potential in the mixed films. The latter effect is attributed to altered lipid dipole orientations and changes in the local dielectric in the mixed films. In contrast to the results at the air/water interface, ferritin adsorption to transferred monolayers, studied using total internal reflection fluorescence, demonstrated decreased ferritin binding on all SME–DOMA mixed films compared to pure DOMA films. These opposite protein adsorption trends on a solid support s. at the air/water interface are considered in terms of differences in monolayer packing, fluidity and phase behavior.