A charged particle passing through water creates a thermal spike, a region of high temperature along the track. The thermal spike expands explosively, thus producing a pressure wave, and then breaks up because of surface tension into discrete regions of water vapor and hydrogen gas. These vapor-gas microbubbles can act as nucleation centers in superheated or gas supersaturated solutions. Calculations based on this thermal spike model are presented of the total energy and minimum linear energy transfer (LET) required to form nucleation centers of a given size, and the calculations are compared to published data on the radiation nucleation of superheated and supersaturated aqueous solutions. Calculations are also presented of the pressure created by the rapid expansion of the thermal spike, and of the lifetime of the vapor-gas microbubbles under conditions in which they collapse. The calculations cover an LET range of 0.1 to 10 Mev/µ or, approximately, from the maximum LET of recoil protons in water to the maximum LET of fission fragments in water. The calculations are carried out for a liquid pressure of one atmosphere and two temperature conditions : the minimum temperature at which vapor nuclei of given size will grow and 0°C. The effect of high pressures and temperatures on the radiation nucleation of vapor bubbles is discussed briefly in terms of the foam limit.