31P nuclear magnetic resonance (n.m.r.) of highly oriented NaDNA and LiDNA fibers was measured as a function of relative humidity over the range from 66% to 98%. The humidity dependence of the spectral patterns of NaDNA fibers shows that the A form has a single conformation while the B form has multiple conformations, and that interconversion between the A and B conformers in the transition region is slow compared to the n.m.r. time-scale (approximately 10(-5) s). Two major conformations of the B form of LiDNA are found to be stable at low relative humidity and they rapidly interchange at high relative humidities. The spectral patterns of immobilized LiDNA are compatible with the single-crystal structure of double-stranded oligo nucleic acids.