The potentiometric properties of the rigidly cross-linked Sephadex gel CM-25 have been studied. The gel was equilibrated in sodium perchlorate at fixed concentration levels (0.010 and 0.0010M) and with variable concentration levels of NaClO4 (from 0.0010 to 0.10M). The volume of the gel was measured for every equilibrated sample simultaneously with the solution pH and pNa. The pK computed after correction for a Donnan potential term was found to be independent of α, the degree of neutralization, and ionic strength; its value of 3.35 ±0.15 was in excellent agreement with the intrinsic pK measured earlier for the flexible CM-50 Sephadex gel and reported for its linear polyelectrolyte analogue, carboxymethyldextran. An interesting observation from these studies is the efficient screening of the charged surface by the counter-ions in the presence of a three-dimensional charged network. © 1984.