Kinetic analysis of the early, fast reaction of superhelical DNA with formaldehyde reveals that this region or regions is 56% "single strand like" in character. Hydrogen-tritium exchange studies coupled with other considerations show that this reaction is not due to a difference in conformational motility between form I and form II molecules, but is due to unpaired or weakly hydrogen bonded, localized region(s) of the form I allomorph of circular DNA.