We present a simulation of liquid water radiolysis by swift carbon ions that explicitly takes into account multiple ionization of water molecules. For high linear energy transfer (LET), this process is not negligible with respect to single ionization. The rearrangement of highly ionized water molecules is consistent with production of atomic oxygen. Multiple ionizations is shown to be responsible for creation of a large amount of HO2/O-2(-) radicals and O-2 molecules. The simulated HO2/O-2(-) yield reproduces very well direct optical measurements for swift ions with comparable LET. Our simulation for O-2 + HO2/O-2(-) is in also agreement with experiment. (c) 2005 Elsevier B.V. All rights reserved.