Various effects that liquid and solid media produce on the experimental activation energy of a reaction involving a single chemical step are discussed. The effects are experimentally observed as significant variations in the activation energy with temperature or extent of reaction. Since these phenomena are inconsistent with the concept of a constant activation energy, that of variable activation energy is considered as an alternative. The discussion covers the kinetics of such processes as electron transfer, liquid phase polymerization, desorption from solids, and solid state decomposition.