A range of linear charge densities of the ordered and disordered forms of DNA or polynucleotides can be obtained experimentally by acid or alkaline titration, or by the investigation of unusual complexes involving protonated bases or three‐stranded helices. The variation of melting temperatures with Na+ concentration for various of these systems is known and in some cases is complemented by structural and thermodynamic information. We have extended the condensation–screening theory of Manning [Biopolymers, 11, 937–955 (1972)] to these systems. The stabilizing and destabilizing effects of Na+ (condensation and screening, respectively) and be independently varied, and the theory is successful in predicting the qualitative (in some cases, quanittative) behaviour that is observed. Comparison of theory and experiment indicates that the axial phosphate distance b for single‐stranded polynucleotides increases with increasing pH. Values of the critical parameter ξ are obtained for the various polynucleotide structures. These values are essential for an understanding of ionic effects on charged ligand–polynucleotide interactions.