Evidence has been reported which indicates that dilution of a salt free DNA solution below a critical concentration zone (4–50 · 10−5M) results in denaturation. This process is irreversible with respect to the specific hydrogen bonds present in the native structure, but is thought to be partially reversible with respect to non-specific hydrogen bonds. The non-specific hydrogen bonds would form above the critical concentration zone between polynucleotide strands at points of close approach. This latter state can be reached by reconcentration to above the critical concentration zone and also by addition of salt to solutions below the critical zone. The structural change resulting from dilution below the critical concentration zone appears similar to that observed for saline DNA solutions which undergo denaturation when heated through a critical temperature range. In both instances the broad ranges of denaturation constraint can be interpreted in terms of hydrogen bond strength heterogeneity.