The densities of aqueous solutions of ((dodecyloxy)methyl)-18-crown-6 (C-12-OM-crown) were measured at various temperatures (5-45 degrees C). The partial molar volume of the micelles at infinite dilution (V-m degrees) in water increased linearly with the temperature below the cloud point (t(c) = 30.5 degrees C) of aqueous C-12-OM-crown solutions from 443.7 cm(3)/mol at 5 degrees C to 452.9 cm(3)/mol at 25 degrees C. C-12-OM-crown micelles seemed to be more bulky than micelles of conventional nonionic surfactants having an oligo(oxyethylene) chain, because of a cyclization effect of the chain. V-m degrees increased upon the addition of KCl. In 0.5 mol/dm(3) KCl systems, the V-m degrees-temperature (T) plot comprised two linear sections with a break point at around 25 degrees C, one of which at over 25 degrees C nearly agreed with the extrapolated line of the V-m degrees-T relation for a no-salt system. The agreement seems to imply a certain compensation effect on the molar volume of partially chelated mixed micelles, since the difference (4.1 cm(3)/mol) in V-m degrees at 5 degrees C between water and 0.5 mol/dm(3) KCl systems may be fairly well interpreted by K+ chelation of the head group of the C-12-OM-crown. The electrostatic interactions between partially chelated mixed micelles may be one of the causes of the concentration-dependent molar volume in KCl systems. Wormlike cylinder micelles of C-12-OM-crown in water at 25 degrees C have a persistence length of about 20 nm, and the interaction between the flexible cylindrical micelles may lead to phase separation. The attractive contribution to the micelle-water interaction may predict the temperature of the phase transition.