"The geochemistry of chlorine isotopes" describes Hans Eggenkamp's studies to obtain his PhD degree at Utrecht University. It describes methods to measure stable chlorine isotope variations in different types of natural samples, theoretical fractionation as the result of diffusion and about ten case studies showing chlorine isotope fractionation in natural systems. In these case studies it was discovered that chlorine stable isotopes fractionate only little, although systematically, in processes such as salt precipitation and diffusion. Variations found in natural samples reflect as such in most cases processes that take place within (larger) reservoirs. The main purpose of the project was to develop procedures to measure stable chlorine isotope compositions in different sample types and to study variations therein. The study gives a good overview of the knowledge on stable chlorine isotope geochemistry at the moment of its original publication.