Changes in vibrational absorption and vibrational circular dichroism (VCD) spectra of polyribocytidylic acid (polyC) in buffered D2O solution as a function of pH and temperature are reported. Analysis of these spectral data led us to establish the absorption band at similar to 1693 cm(-1) and associated negative VCD couplet as diagnostic markers of the double-helical form of polyC. An alternate interpretation suggesting quadruplex formation for polyC is also discussed. In addition to the solution state spectral data, pH-dependent absorption and VCD spectra for polyC films derived from dilute H2O solutions are also presented. The pH-dependent changes in the absorption and VCD spectra of polyC films are found to be similar to those observed for polyC in solution.