1. Reactions in solution that go as fast as possible are called diffusion controlled, because every time the molecules find each other by random collision, they react. We have studied an example of such fast reactions in which a halogen (such as fluorine, chlorine, bromine, or iodine) of a light-excited molecule is substituted by a group coming in with a pair of electrons to make a bond. The rates of these reactions are influenced by the atomic environment at the reaction site. The ability of the halogen to leave as the anion is irrelevant to how fast the reactions occur. The hindrance of halogen size to attack of the incoming group and the positive effect of polarization of the bond by halogen are what govern these very fast reactions. The light-caused reactions are about 14 powers of ten faster than the corresponding non-light or thermal reactions. It is astounding that the relative influences of the four halogens and of the power to make a bond of the incoming group are just about the same for the light-caused reactions as for the thermal reactions. This implies that the geometry and nature of the crucial bond-forming steps in the two reaction types are very similar despite their vast differences in rate.