The proportion of 18O in a sample of seawater is an excellent tracer of its past history as, away from the surface, it is conservative and also nondynamical. The range of values in source waters is also large, and the accuracy achievable in modern measurement high. Here we bring together for the first time a global data set of over 6000 individual measurements from the past 40 years. The properties of this data set are described. Noteworthy features include the hitherto unnoticed, but distinctive, contribution of North Pacific Upper Water to the δ18O:salinity relationship, and different origins of the deeper waters of the Atlantic and Pacific Oceans. We also make a plea here to the international community to contribute unpublished data to the archive for the use of all.