Telomeres are structures at the ends of eukaryotic chromosomes, the DNA of which contains stretches of tandemly repeated sequences with G clusters along one strand. Model telomeric G-rich DNAs can form different tetraplex structures, stabilized by cyclic hydrogen bonding of four guanines in the presence of metal ions such as Na+ or K+. Oligonucleotides with a single copy of the Oxytricha sequence dT4G4 form a tetramer, with a parallel-stranded, right-handed helical structure. Additional copies favor folded-back structures that associate to form an antiparallel dimer. The parallel-stranded tetramer has all G's in the anti configuration, while the folded-back dimer has alternating syn and anti nucleotide conformations along each strand. Here we have constructed two G-tetraplex structures, containing identical G-tetrad base pairs, from oligonucleotides. One has the truncated telomeric sequence from Oxytricha, dG4T4G4, which forms an antiparallel G-quartet structure; the second is constrained to form a parallel G-strand arrangement by insertion of a 5'-p-5' linkage between two dT2G4 sequences. Each oligomer forms a defined G-tetraplex dimeric structure in the presence of Na+. The standard-state enthalpies, entropies, and free energy for formation of these tetraplexes have been determined. The parallel strand structure is thermodynamically more stable than the antiparallel one, primarily because of both greater enthalpy and entropy of formation. In addition, the two molecules differ in their interaction with sodium ions, reflecting a difference in ion binding and therefore in structure between the two forms.