Eighty years of chlorine atomic weight measurements revealed no variation of the stable isotope ratio, 37C1/ 35C1 (with precision up to 1.0%.) in natural materials. This result is not surprising because chlorine occurs in relatively few compounds, has a strong affinity for the liquid phase, occurs mostly in the -1 oxidation state and organisms don't discriminate between chlorine isotopes. Chlorine isotopes have been found to fractionate in the laboratory during kinetic reactions, equilibrium between phases and diffusion. This dissertation examined chlorine isotope composition of chloride from sea water halite, hydrothermal water samples and ground-water samples where chloride was likely moving by diffusion. The measurement method was mass spectrometry of methyl chloride gas prepared by quantitative precipitation of AgC1 from solution, and reaction of the AgC1 with methyl iodide. The precision of the technique is 0.24%. Results from sea water indicate that isotope ratios in sea water do not vary beyond measured uncertainty, thus sea water became the designated standard called SMOC (Standard mean ocean chloride). Many of the samples measured in this study vary significantly, though most are within 1.0%, of SMOC. All halite and hydrothermal samples are heavier than SMOC. Halite results may indicate isotope effects durina precipitation or time dependent variations of sea water. Hydrothermal samples may indicate source differences and/or fractionation mechanisms. Milk River aquifer samples indicate that chloride in the same aquifer may vary due to dual filtration or differina sources. Samples from a Canadian glacial clay show a total chloride variation of 2.4%,, linearly distributed with depth. chloride in the clay is known to be diffusing against ground-water movement. The isotope distribution can be approximated with a counter-current column equation. Samples from Texas and Louisiana contain an isotope range of about 1.8%, linearly distributed with depths from 6,000 to 14,000 feet. The distribution can be approximated with a simple diffusion equation. As a tracing tool, chlorine isotope measurements showed that halite AO kilometers from the ocean in south Africa probably did not precipitate from sea water mist, and that oil field brines adiacent to the Weeks Island salt dome (Louisiana) probably did not receive substantial quantities dome chloride.