A comparative study of the acid-base and copper(II) complex-forming properties of the homopolyribonucleotides polyuridylic acid, polyinosinic acid, polycytidylic acid and polyadenylic acid has been carried out. The results show that the differences in the conformational changes (from single stranded to higher ordered structures, such as double stranded helices) and polyelectrolytic behaviour that are observed in these polynucleotides in aqueous solution when pH is varied are mainly due to differences in the nature of the functional groups involved in the acid-base equilibrium for each nitrogen base. Copper(II) complexation also plays an important role in these macromolecules conformation. Instead, the purine or pyrimidine nature of the nitrogen bases has only a relative importance and mainly in connection to the ability of stacking between bases.