The aqueous solutions of hydrochloric acid-tetraethylene glycol monooctyl ether (C8E4) and sodium chloride-C8E4 mixtures were investigated to examine the interaction between inorganic ions and nonionic surfactants both in the adsorbed film and micelle. Their surface tension was measured as a function of the total molality of the inorganic electrolyte and C8E4 mixture and the composition of C8E4 The results of surface tension measurement were analyzed by our thermodynamic procedure, and the phase diagrams of adsorption and micelle formation were drawn. By comparing the phase diagrams of these systems, it was been that the Na+ ions do but Cl- ions do not interact with the ethylene oxide group of the C8E4 molecule. This finding leads us to the conclusion that the attractive interaction observed in the dodecylammonium chloride-C8E4 system reported previously is caused by ion-dipole interaction or hydrogen bonding between the dodecylammonium ion and oxygen atom of the ethylene oxide group of the C8E4 molecule.