The aqueous phase behavior of heptacthylene glycol dodecyl ether (C12E7) was investigated in the presence of sodium salts of Cl-, I-, and ClO4. Pseudo binary T-X phase diagrams were constructed for these mixtures by means of differential scanning calorimetry. The salting-out electrolyte NaCl expanded the L-alpha region toward higher temperatures and shrank the H-1 region toward lower temperatures compared with the salt-free system. On the contrary, the salting-in electrolytes NaI and NaClO4 induced shrinkage of the L-alpha region and an expansion of the H-1 phase. The influence of these salts on the mesophase regions was more pronounced for the L-alpha phase than for the H-1 phase, and area of the L-alpha phase region decreased in the sequence of NaCl > none > NaI > NaClO4, consist with the Hofmeister series of the anions. This salt effect on the mesophase stability in aqueous nonionic surfactant, mixture would be qualitatively interpreted in terms of the salt effect on the hydration of the polyoxyethylene chain in the surfactant molecules. (C) 2004 Elsevier Inc. All rights reserved.