4-[4(-(4(-decyloxy phenylazo) naphthloxy] butyl trimethylammonium bromide can form a stable monolayer on the subphase of an aqueous sodium dextran sulfate solution. Infrared transmission, reflection-absorption and ultraviolet-visible spectra have been measured for the one- and multi-layer LB films to study the molecular orientation and aggregation in the films. It was found that the title compound does not form molecular aggregation in the LB films. The alkyl tail is nearly perpendicular to the substrate surface and adopts largely trans-zigzag conformation.