Data of activity of individual ions reported in the literature by nine authors are compared in graphical form. Visual observation of the plots clearly shows that for some systems the data are in fair agreement and it can be used to test theories of electrolyte solutions. For systems that the data show discrepancy between different researchers, it is possible to judge which data are out of the trend showed by the majority of the other studies. Only a few systems appear to need further measurements. This compilation of results is the first of its class in modern times and not only helps in showing the consistency between data from different laboratories but it also indicates for what systems data are still needed. (C) 2016 Elsevier Ltd. All rights reserved.