We report accurate measurements of the mobility of excess electrons in high-density helium gas in extended ranges of temperature [(26 < or = T < or = 77) K] and density [(0.05 < or = N < or = 10.0) atoms nm(-3)]. The aim is the investigation of the combined effect of temperature and density on the formation and dynamics of localized electron states. The main result of the experiment is that the formation of localized states essentially depends on the relative balance of fluid dilation energy, repulsive electron-atom interaction energy, and thermal energy. As a consequence, the onset of localization depends on the medium disorder through gas temperature and density. The transition from delocalized to localized states shifts to larger densities as temperature is increased. This behavior can be understood in terms of a simple model of electron self-trapping in a spherically symmetric square well.