In 2024 it was 40 years ago that the first major work on chlorine isotopes was published (Kaufmann 1984). In this PhD thesis Ronald Kaufmann described the first method with which it was possible to measure the small natural variations of chlorine stable isotopes that appear in groundwaters and applied the technique to better understand the hydrological cycle of the chloride ion. Considering the fact that no chlorine isotope variations in natural samples were known before Ronald Kaufmann started his PhD studies the research focussed on ground water for the following three reasons: (1) The only samples that (possibly) showed natural variations in the study by Hoering and Parker (1961) were ground waters; (2) Groundwater environments are diverse and many locations might have fractionated chlorine due to processes that could be understood at that time; (3) Chloride could be removed quantitatively from water samples through a relatively simple technique by precipitating chloride with silver ions as silver chloride. Kaufmann (1984) studied chlorine isotope variations in different types of water samples, such as hydrothermal waters, formation waters and aquifer samples. Kaufmann (1984) also analysed a few halite samples. These early measurements showed clearly that the chlorine isotope ratio varied by at least 2.1‰, with the observation of values between −1.3 and + 0.8‰ relative to SMOC, a much larger range than was seen before and probably also a much larger variation than was expected before the research started (Kaufmann et al. 1984). Interestingly in oil field brine samples the δ37Cl values showed an increasing trend with depth and it was suggested that this enrichment with depth might reflect an isotope fractionation mechanism related to the electrostatic properties of subsurface clays. These early studies further showed positive evidence that chlorine isotope variations could be observed in studies on groundwater diffusion (Desaulniers et al. 1986) and ion-filtration (Campbell 1985).