Planetary exploration missions to Jupiter and its moons require that the power system components, including batteries, be tolerant to high intensities, about 4 Mrad, of gamma-radiation. In view of the several polymeric materials used as separators and binders and the use of organic electrolyte solutions in Li-ion cells, it is difficult to predict their response to such radiation environments. Therefore, a detailed experimental evaluation was undertaken to determine the performance of Li-ion cells after exposure to various levels of cumulative radiation levels up to 25 Mrad, at different levels of intensities. Prototype cells, obtained from two domestic sources for aerospace lithium-ion batteries and consisting of two different chemistries, were used as test articles. Discharge performances, at ambient and low temperatures, as well as, electrical impedance spectroscopy responses were determined after each exposure, and analyses were made for the impedance characteristics and their changes upon irradiation. Postradiation cycling tests were carried out on these cells to assess their cyclability subsequent to radiation exposure. Although control measurements were not made on cells without radiation, these studies reveal that the lithium-ion cells display good tolerance to radiation, with only marginal decline in their capacity, and with no significant change in capacity fade rate during subsequent cycling. (C) 2004 The Electrochemical Society.