The mechanism for the formation of the M" ion of a-tocopherol, a viscous oily material, under fast atom bombardment (FAB) conditions without a liquid matrix was investigated by using positive and/or negative-ion electron impact, gas-phase FAB, gas-phase fast molecule bombardment and charge-exchange reaction techniques. The results obtained could not be explained by the charge exchange alone, and they also suggested that M" ion formation under FAB conditions is not due to direct collisions between analytes and fast atoms but to collision interactions between analyte molecules in a dense gas-like phase formed immediately after the collision cascade caused by particle bombardment and/or in the collision cascade itself. It is argued that the formation of M" ions under matrix FAB conditions is essentially due to collision interactions between analyte and matrix molecules. In order to understand the M" ion fromation. the energetics are discussed from the standpoint of the charge exchange ionization reaction.