Phase transitions were studied of the sodium salt of poly(rA)·poly(rU) induced by elevated temperature without Ni²⁺ and with Ni²⁺ in 0.07 M concentration in D₂O (~0.4 [Ni]/[P]). The temperature was varied from 20° C to 90° C. The double-stranded conformation of poly(rA)·poly(rU) was observed at room temperature (20° C - 23° C) with and without Ni²⁺ ions. In the absence of Ni²⁺ ions, partial double- to triple-strand transition of poly(rA)·poly(rU) occurred at 58° C, whereas only single-stranded molecules existed at 70° C. While poly(rU) did not display significant helical structure, poly(rA) still maintained some helicity at this temperature. Ni²⁺ ions significantly stabilized the triple-helical structure. The temperature range of the stable triple-helix was between 45° C and 70° C with maximum stability around 53° C. Triple- to single-stranded transition of poly(rA)·poly(rU) occurred around 72° C with loss of base stacking in single-stranded molecules. Stacked or aggregated structures of poly(rA) formed around 86° C. Hysteresis took place in the presence of Ni²⁺ during the reverse transition from the triple-stranded to the double-stranded form upon cooling. Reverse Hoogsteen type of hydrogen-bonding of the third strand in the triplex was suggested to be the most probable model for the triple-helical structure. VCD spectroscopy demonstrated significant advantages over infrared absorption or the related electronic CD spectroscopy.