Pressure-area (pi-A) isotherms were obtained from dioctadecyldimethylammonium chloride (DODAC) monolayers at the air-water interface for a range of subphase salt concentrations. Simultaneous surface potential measurements confirmed the important role of electrostatic repulsion between ionized headgroups in controlling the liquid-expanded to liquid-condensed phase transition. The extrapolated area per molecule measured in the condensed phase, 0.58 nm(2), was in good agreement with previously reported X-ray diffraction measurements on hydrated crystals. The Langmuir-Blodgett (LB) deposition of uniform monolayers of DODAC onto aluminium and gold substrates was investigated by surface potential measurements, phase contrast microscopy and surface plasmon resonance microscopy. Atomic force microscopy confirmed the regular molecular order of the deposited monolayer, yielding a spacing of approximately 0.46 nm between rows of adjacent molecules.