The suitability of gel electrophoresis to structural analysis of nucleic acids has been examined, using from borate buffer concentrations (in the range from 4.5 to 450 mM. The gel electrophoretic mobility of single-stranded oligonucleotides was shown to be sequence-dependent at higher concentrations than 27 mM of borate buffer and nondependent at lower one (less than 9 mM). As a result, each dodecamer had a sequence-specific critical concentration of Tris-borate-EDTA (TBE) buffer at which each seems to change its structural state. At the lower concentration than the critical one, all the dodecamers turned to the state of a finite mobility and migrated in a sharp band. This finding is discussed and rationalized by the assumption that the difference in conformational dynamics of oligonucleotides, due to the difference in their sequence, is mainly responsible for the observed difference in their mobility.