The anion exchange fibers on the basis of industrially produced polyacrylonitrile fiber NITRON with different strong and weak base groups have been described in the paper. Their acid-base properties were characterized by the acidity parameters calculated from the potentiometric titration curves. The method of computer analysis of the titration curves described elsewhere was used. It was established that the ion exchange fibers are polyfunctional and contain at least four types of functional groups. Their capacities and the acidity parameters have been calculated. The fibers have full ion exchange capacity about 3.5 meq/g. The capacity according to the strong base quaternary aminogroups (pK congruent to 2) is usually about 2.5 meq/g. The water uptake, stability in hydrogen peroxide solutions, strong acids and alkalis of different concentrations at the ambient temperature and 100 degrees C have been studied. The fibers do not loose significantly their anion exchange capacities and mechanical properties except for the most extreme regimes of their treatment. At the same time they accumulate carboxylic acid groups in their structure due to hydrolysis of the residual nitrile groups in the fiber. (c) 2005 Elsevier B.V. All rights reserved.