Diffusion probably is the most well known and most well understood process responsible for variations in chlorine and bromine isotope compositions. Molecular diffusion is the process in which matter is transported from one part of a system to another as a result of arbitrary molecular movements (Crank 1956). It was first described by Fick (1855). His work is now referred to as Fick’s First and Second Laws, and was published even before quantitative experimental measurements had ever been done. Although Lindemann (1921) discussed the possibility that, due to the mass difference of isotopes from a single element isotopes could theoretically be separated electrolytically, it must be realised that diffusion experiments to separate potential (because still undiscovered) isotopes of chlorine were started already in 1915 (Harkins and Hall 1916, p. 221) even before the actual discovery to the chlorine isotopes by Aston (1919). Experimental separation of chlorine isotopes by diffusion was shown to exist in 1920 (Harkins 1920a, 1920b, Harkins and Hayes 1921; Harkins and Ligget 1924; Harkins and Jenkins 1926). In these studies, set up to confirm that chlorine exists of several isotopes, no attempts were made yet to quantify the fractionation factor of diffusion, although estimates were made to calculate on what scale diffusion would separate the chlorine isotopes as compared to neon. The first study in which fractionation due to diffusion was well quantified was published in the 1940s (Madorsky and Straus 1948) and a variety of studies on diffusion of chlorine (and bromine) isotopes have been published since then, consisting of theoretical, experimental and field studies.