An unusual reversible effect was observed in the synthesis of CdSe nanoparticles in the aqueous tertiary butanol solutions containing equimolar Cd[NH3](4)SO4 and Na2SeSO3 by the high-energy electron beam irradiation. These nanoparticles were found to be unstable in the aqueous solutions and decomposed upon exposure to air/oxygen. However, they were stable in the de-aerated aqueous solutions and both aerated and de-aerated organic solvents. The decomposed aqueous solutions again produced these nanoparticles upon irradiation with the electron beam. On the contrary, the cadmium selenide nanoparticles synthesized with the cobalt-60 gamma-irradiation, were stable under ambient conditions and did not exhibit any reversible behavior. This was attributed to the vast differences in the dose rates involved in the two types of synthetic processes, leading to a remarkable variation in the crystal structure and size of the nanoparticles. (C) 2010 Elsevier B.V. All rights reserved.