Research on the counterion binding equilibria of the linear polyelectrolytes with anionic groups, i.e., calboxyl, sulfate, or phosphate group, is important for understanding the effect of the electrostatic interaction on the functionalities of polyions. For the analysis of the counterion binding equilibria of polyelectrolytes, the electrostatic interaction between the counterions and the ionic groups fixed on the polymer skeletons should be quantitatively evaluated. In this research, a "two-phase model" which the formation of a polyelectrolyte phase was assumed around the polymer skeleton was applied. In our previous study, it has been confirmed that the Donnan potential and the polyelectrolyte phase volume of weak acidic polyions can be expressed as a function of the average linear charge distance on the polyions. In this work, the divalent metal ion binding equilibria of dextran sulfate anions of different degree of substitution as typical strong acidic polyions was investigated in order to clarify the relationship between the polyelectrolyte phase volume, V(p), and the average linear charge distance of the negative charge fixed on the strong acidic polyion skeletons. The polyelectrolyte phase volume per fixed ionic group on a polyelectrolyte skeleton, V(p)/n(p), which was estimated by the application of the two-phase model showed a nonmonotonic change against the average linear charge distance, 1/b. The V(p)/n(p) increased drastically in 0 < 1/b < 0.2 angstrom(-1) and showed the local maximum at 1/b = ca. 0.25 angstrom(-1) The obtained findings are expected to provide important information for the molecular design of functional polyions of various shapes.