The osmotic coefficient g* is calculated for sodium carboxymethylcellulose (NaCMC) samples of different degrees of substitution. The dependence of g* onDS, concentration of NaCMC, and added NaCl has been investigated for all samples. The true osmotic coefficient has also been calculated for all the samples by applying Donnan's equilibrium concept. is found to be independent of the amount of extraneous salt accounting for Donnan's membrane equilibrium. The dependence of on the charge density of polyions can be explained quantitatively by Oosawa's and Katchalsky's theories.