The hybridization of DNA by sequential electrostatic and hydrogen-bonding immobilization of single-stranded complementary oligonucleotides at the air-water interface with cationic Langmuir monolayers is demonstrated. The complexation of the single-stranded DNA molecules with octadecylamine (ODA) Langmuir monolayers was followed in time by monitoring the pressure-area isotherms. A large (and slow) expansion of the ODA monolayer was observed during each stage of complexation in the following sequence: primary single-stranded DNA followed by complementary single-stranded DNA followed by the intercalator, ethidium bromide. Langmuir-Blodgett (LB) films of the ODA-DNA complex were formed on different substrates and characterized using quartz-crystal microgravimetry (QCM), Fourier transform infrared spectroscopy (FTIR), polarized-UV-vis and fluorescence spectroscopy, as well as thermal melting studies. These measurements clearly showed that hybridization of the complementary single-stranded DNA molecules had occurred at the air-water interface, leading to the characteristic double-helical structure. Furthermore, it was observed that the DNA molecules in the LB films were oriented parallel to the substrate withdrawal direction.