The molecular organization of mixed monolayers containing aminostyryl-pyridinium dyes of different alkyl chain lengths and stearic acid was studied at the air-water interface and on solid substrates. The surface pressure-area and surface potential-area isotherms indicate that the molecular organization in these mixed films is controlled by the alkyl chains. Polarized angular reflection measurements at the air-water interface provide evidence for the presence of two dye populations with different orientations. Mixed monolayers on hydrophobized glass substrates transferred via vertical dipping show analogous results. In the case of the short-chain dye, H aggregation is observed together with monomer chromophores with different orientations. Furthermore, non-ideal mixing of dye and matrix is found in this case.