The mobility of positive ions was measured in supercritical ethane, xenon, and carbon dioxide as a function of pressure at several temperatures. In supercritical CO2 the mobility of negative ions was also measured. Radii of the moving clusters were calculated with a hydrodynamic compressible continuum model that takes the enhancement in density and viscosity near the ions into account. The results show that positive ions move with a large solvation shell. Changes in size of this shell occur as a function of temperature and pressure. The largest radii at each temperature are found to occur at the pressure corresponding to the maximum in the isothermal compressibility. For positive ions the first solvation shell is totally filled under all conditions.