The properties of the complex monolayers composed of cationic gemini surfactants, [C18H37(CH3)(2)N+-(CH2)(s)-N+ (CH3)(2)C18H37],2Br(-) (18-s-18 with s = 3, 4, 6, 8, 10 and 12), and ds-DNA or ss-DNA at the air/water interface were in situ studied by the surface pressure area per molecule (pi-A) isotherm measurement and the infrared reflection absorption spectroscopy (IRRAS). The corresponding Langmuir-Blodgett (LB) films were also investigated by the atomic force microscopy (AFM), the Fourier transform infrared spectroscopy (FT-IR), and the circular dichroism spectroscopy (CD). The pi-A isotherms and AFM images reveal that the spacer of gemini surfactant has a significant effect on the surface properties of the complex monolayers. As s <= 6, the gemini/ds-DNA complex monolayers can both laterally and normally aggregate to form fibril structures with heights of 2.0-7.0 nm and widths of from several tens to similar to 300 nm. As s > 6, they can laterally condense to form the platform structure with about 1.4 nm height. Nevertheless, FT-IR, IRRAS, and CD spectra, as well as AFM images, suggest that DNA retains its double-stranded character when complexed. This is very important and meaningful for gene therapy because it is crucial to maintain the extracellular genes undamaged to obtain a high transfection efficiency. In addition, when s <= 6, the gemini/ds-DNA complex monolayers can experience a transition of DNA molecule from the double-stranded helical structure to a typical psi-phase with a supramolecular chiral order.