Under transition state theory, a reaction coordinate diagram is a way of visualizing a reaction pathway. A reaction coordinate diagram represents a pathway that traces saddle points along a hypothetical three-dimensional energy surface describing the vibrations, rotations, and translations of bonded atoms, leading from the configuration of the reagents through that of the transition state to the configuration of the products.

The reaction coordinate follows the path of free energy from reactants to products. The pathway corresponds to breaking and forming new bonds. The rate of passage over the barrier is influenced by the height of the height of the barrier (activation energy) as well as the energy distribution (temperature).