Hofmeister, or lyotropic, series date back to 1888, when the founder arranged a large number of electrolytes in sequences with respect to their effectiveness salting out egg white. Since then the name has been applied to various phenomena involving ion specificity. In order to isolate effects attributable to single ionic species an interpretational step has to be taken, because only effects of electroneutral combinations of ions can be measured. The resulting sequence depends on the particular phenomenon studied. As a result, a variety of observations have been reported without a clear organizing principle. Here we describe Hofmeister sequences for well-defined systems which allow clear definition and unambiguous interpretation. Hofmeister sequences are not unique, they depend on the system and are the result of pair interactions. (c) 2008 Elsevier B. V. All rights reserved.