Abstract The dependence of reduced dichroism (Δε/ε) of sonicated rodlike title triplexes with the counterion Mg2+, (dA)n·(dT)n·(dT′)n and (rA)n·(rU)n·(rU′)n, on the wavelength (190—300 nm) and on the applied-field strength (3.8—23.1 kV cm−1) was studied in aqueous ionic solutions at 7 °C. The sonicated samples were fractionated and well-characterized by the gel-permeation chromatography/low-angle laser light-scattering method. The reduced dichroism saturated at infinitely high electric fields ((Δε/ε)s) was estimated to be −1.22 for (dA)n·(dT)n·(dT′)n and −1.09 for (rA)n·(rU)n·(rU′)n, by fitting the observed Δε/ε values at 260 nm to a theoretical orientation function. The electric linear dichroism (ELD) spectra, i.e., the dependence of (Δε/ε)s on the wavelength, of (dA)n·(dT)n·(dT′)n and (rA)n·(rU)n·(rU′)n were determined. The measured ELD spectra were simulated by assigning the appropriate roll (θR) and tilt (θT) angles to the constituent bases. The most probable θR and θT values, determined for each triplex, indicate that the structures of triple helices both belong to an “A-form” family. The constituent bases in base-triple do not lie on a common plane. From an analysis of transient decay signals, the translation per base-triple (h) was estimated for each triplex. The structures of the present high molecular-weight triple-stranded helices in aqueous solution were determined for the first time and the plausible structural models were constructed from the experimentally evaluated sets of θR and θT.