Comparison between gel electrophoresis migrations of oligo-α-thymidylates and oligo-β-thymidylates indicates that the migration of α-oligonucleotides under native conditions is different from the migration of β-oligonucleotides when the number of thymines, part of the sequence, is higher than 5. Such difference disappears when the gels are run under denaturing conditions. This, together with UV spectra, indicates that the structure of α-oligonucleotides is more organized than the structure of β-oligonucleotides and that such an organisation appears for a length than 5 monomeric units.