Aggregation behaviour of Chitosan and glycol-Chitosan stabilized silver nanoparticles was investigated over the pH range of 3.4-13. With increasing pH, the charge of the particle reduces and approaches zero beyond 6.5 (the pK(a) of the amino group) inducing aggregation that was confirmed by absorption spectroscopy, hyper-Rayleigh scattering (HRS) and TEM measurements. Interestingly the HRS intensity increased till pH 9.0 +/- 2.0 and beyond this it dropped dramatically. This correlated with the formation of centrosymmetric spherical aggregates as evidenced by TEM pictures. The aggregation process was found to be reversible over at least 10 pH cycles by absorption and HRS measurements. (C) 2011 Elsevier B.V. All rights reserved.