The acid-base properties and the copper(II)-complexing behaviour of the polynucleotide polycytidylic acid (poly(C)) were studied by means of potentiometric, spectrophotometric and circular dichroism titrations in a working aqueous medium of 0.15 M ionic strength and at 37 degrees C. Spectrophotometric and circular dichroism data were treated with a multivariate curve resolution procedure that is successfully applied to the study of macromolecular systems, where polyfunctional, conformational and/or polyelectrolytic effects may be present. This procedure allows different sets of spectrometric titrations of the multi-equilibria systems to be analyzed simultaneously. Three spectroscopically different species are detected for poly(C) in the pH range 3.1-7.0. Two of them are forms with different protonation extent and different conformation, and the other is the unprotonated form of poly(C). One new species is detected in the presence of Cu(II) ion.