AbstractThe osmotic coefficients ϕp,Na of dilute solutions of the sodium form of some weakly acidic polymers are theoretically predicted in this work. Based on the measured value 0.73 of γNa, the activity coefficient of free Na+, of the completely ionized humic acid (sodium salt) in a salt‐free solution, the effective interligand distance b is calculated to be 11.34 Å by using Manning's counterion condensation theory [Manning, G. S. (1969) J. Chem. Phys. 51(3), 924]. The corresponding values of γNa (measured experimentally) and b for the completely ionized exopolymer of Pseudomonas atlantica are 0.624 and 7.57 Å when cultivated at a dilution rate D = 0.015 h−1, 0.647 and 8.19 Å at D = 0.025 h−1, and 0.613 and 7.29 Åat D = 0.06 h−1. For alginic acid (in the completely ionized sodium form), γNa = 0.40 and b = 4.71 Å. The osmotic coefficients ϕp,Na for the partially and the completely ionized polymers are then predicted with Manning's theory as well.