Triplex formation occurs via interaction of a third strand with the major groove of double-stranded nucleic acid, through Hoogsteen hydrogen bonding. In this work, we use a combination of temperature-dependent UV spectroscopy and differential scanning calorimetry to determine complete thermodynamic profiles for the unfolding of polyadenylic acid (poly(rA))center dot polyuridylic acid (poly(rU)) (duplex) and poly(rA)center dot 2poly(rU) (triplex). Our thermodynamic results are in good agreement with the much earlier work of Krakauer and Sturtevant using only UV melting techniques. The folding of these two helices yielded an uptake of ions, Delta n(Na+) = 0.15 mol Na+/mol base pair (duplex) and 0.30 mol Na+/mole base triplet (triplex), which are consistent with their polymer behavior and the higher charge density parameter of triple helices. The osmotic stress technique yielded a release of structural water, Delta n(W) = 2 mol H2O/mol base pair (duplex unfolding into single strands) and an uptake of structural water, Delta n(W) = 2 mol H2O/mole base pair (triplex unfolding into duplex and a single strand). However, an overall release of electrostricted waters is obtained for the unfolding of both complexes from pressure perturbation calorimetric experiments. In total, the Delta V values obtained for the unfolding of triplex into duplex and a single strand correspond to an immobilization of two structural waters and a release of three electrostricted waters. The Delta V values obtained for the unfolding of duplex into two single strands correspond to the release of two structural waters and the immobilization of four electrostricted water molecules.