This study describes a detailed investigation of swelling and degradation behavior of ionically cross-linked sodium alginate/chitosan bi-polymeric beads in the environment of varying pH to mimic transition of oral dosage form from mouth to colon. The bead sample S-1, prepared by complex coacervation was found to be stable for nearly 320 +/- 12 min in the media of varying pH. The variation in amount of chitosan or cross-linking with Al3+ ions did not result in any enhancement in their stability. However, using 6% sodium alginate solution the beads could be stabilized for nearly 10 h. The formation of multilayer beads (S-2,S-3 and S-4) resulted in an drastic increase in the stability of beads. The mechanism of water uptake, as studied for the sample S, indicated ion exchange process between Ca2+ ions present in polyguluronate and polymannuronate blocks of alginate and Na+ ions present in phosphate buffer medium. This was further confirmed by linear nature of plot obtained between fractional water uptake and time(1/2). (c) 2005 Elsevier B.V. All rights reserved.