The sensitivity of DNA base-pair stacking to water content, as shown here, may be responsible for widely different transport results reported in the literature. Oriented films of a DNA-surfactant complex in which aligned double helices are separated by 4.1 nm, with face-to-face base-pair stacking, bases separated by 0.34 nm, and good pi-pi overlap, are examined. Dehydration drives the base pairs from parallel planar packing to edge packing with poor pi-pi overlap (see Figure).