The interaction with divalent cations of the synthetic polynucleotides, polyadenylic acid and polyuridylic acid, has been studied by conductiometric titration. It is shown that further binding of divalent cation by polynucleotide ceases when one equivalent of cation has been added per mole of polymer phosphate, and that the titration curve is the same for both polymers. This result is compared with that obtained previously for DNA in distilled water, and it is shown that the anomalous end-point for DNA is eliminated if sufficient points are taken in the course of the titration.