The lyotropic behavior of an amphiphilic macrocrown ether (MC-12) with an average ring size of 42 atoms attached to a hydrophobic 12-carbon tail was studied in 70 wt % toluene in the presence of small amounts of water by a combination of polarized optical microscopy, H-1-NMR, X-ray diffraction, and C-13-NMR relaxation studies and by comparison to the lyotropic behavior of the corresponding solutions of Igepal-CO-720 [oligo(oxyethylene) nonylphenyl ether]. Igepal-CO-720 is a classic linear surfactant with a distribution of oligo(oxyethylene) lengths. MC-12 is a macrocyclic surfactant with a distribution of ring sizes. Since it was synthesized from poly(ethylene glycol) 600, 3,4-(42-crown-14)benzyl dodecyl ether is the predominant structure. Similar to Igepal-CO-720, MC-12 in 70 wt % toluene organizes into a lyotropic lamellar (L-alpha) mesophase, in which the macrocrown ethers are relatively flexible, when [H2O]: [EO] less than or equal to 2.2. When [H2O]:[EO] > 2.7, the solutions macroscopically phase separate into an isotropic toluene layer containing only MC-12 and a water-enriched dense white emulsion in which MC-12 is organized in an L-alpha mesophase with its alkyl substituents interdigitated.