The barrierless nature of radical-radical reactions correlates with a large variation in the location and form of the transition state dividing surface with excess energy. In particular, at low temperatures, enthalpic effects are most important and the transition state lies at large radical-radical separations. At higher temperatures, the decrease in entropy with decreasing separation pushes the transition state towards shorter separations. Within variational transition-state theory (VTST), the variationally optimal estimate for the reactive flux is provided by its minimum with respect to some reaction coordinate such as the separation distance.