In applying chemical knowledge to the solution of natural terrestrial problems it is necessary to know the abundances of the chemical and isotopic components of the systems being studied. Broader studies in geochemistry require knowledge of such abundances in various major reservoirs (lithosphere, hydrosphere, atmosphere) and of the mutual exchanges which occur. Concomitant problems are the origins of particular isotopic species, their fates within each reservoir and variations in their budgets with time. While these topics warrant study in their own right, a desirable outcome is the application of such knowledge to related disciplines. This study is firstly an attempt to establish a routine laboratory for the extraction and measurement of geochemical 3 tritium, 3H or T, an isotope of hydrogen. Experimental techniques for the determination of tritium levels in natural waters are described. These include one stage conversion of water to methane and significant improvements in electrolytic procedures for isotopic enrichment. Secondly, water samples from areas of the Pacific and Antarctic Oceans have been collected and analysed for tritium. Conventional hydrographic data have been used in conjunction with the tritium results to determine the physical oceanography of these areas with emphasis on water mass structure and mixing characteristics.