The effect of salts on the nonionic surfactant octoxynol 9 was studied by the changes they produced in its cloud point (CP) : CP increases indicate salting in. The temperatures by which the sodium salts of water structure-breaking (chaotropic) anions increased the CP of 2.0% octoxynol solutions were measured as a function of salt concentration. The curves representing changes in CP versus salt molality rose to a maximum in a parabolic fashion, followed by steep decreases. Their ascending branches, corresponding to salting in, were caused by a disruption of the water structure due to the chaotropic effect of the anions combined with the effect of elevated CP temperatures. The descending branches were due to salting out by Na+. The net CP increases due to the chaotropic effect of the anions were calculated at each concentration by subtracting the CP decrease due to Na+ from the observed CP increase of the respective Na+ salts. With the exception of ClO4-, the plots of CP changes produced by the chaotropic anions rose in a nearly linear fashion to a maximum and then levelled off. The levelling off occurred at the salt concentration and CP temperature leading to the maximum disruption of the water structure of which each anion was capable. The chaotropic anions were ranked in the following order according to their capacity for increasing the CP : SCN- > I- > [Fe(CN)(5)NO](2-) > ClO4- > BF4-. Even though the thiosulfate anion is a very soft Lewis base, it lowered the CP in direct proportion to its molality; i.e., it enhanced the structure of water and promoted salting out at all concentrations.