The first studies in which chlorine isotope variations in relationship with organic compounds were experimentally determined were applied during the 1950s. Probably the earliest of these studies were done by Bartholomew et al. (1954a, 1954b) who studied the chlorine isotope effect in reactions of 2-chloro-2-methyl-propane (tert-butyl chloride). In these studies essentially the isotope effect of the reaction