We have investigated the physical and biomimetic properties of a sponge (L3) phase composed of pentaethylene glycol monododecyl ether (C12E5), a nonionic surfactant, an aqueous solvent, and a cosurfactant. The following cosurfactants, commonly used for solubilizing membrane proteins, were incorporated: n-octyl-beta-d-glucopyranoside (beta-OG), n-dodecyl-beta-d-maltopyranoside (DDM), 4-cyclohexyl-1-butyl-beta-d-maltoside (CYMAL-4), and 5-cyclohexyl-1-pentyl-beta-d-maltoside (CYMAL-5). Partial phase diagrams of these systems were created. The L3 phase was characterized using crossed polarizers, diffusion of a fluorescent probe by fluorescence recovery after pattern photobleaching (FRAPP), and freeze fracture electron microscopy (FFEM). By varying the hydration of the phase, we were able to tune the distance between adjacent bilayers. The characteristic distance (db) of the phase was obtained from small angle scattering (SAXS/SANS) as well as from FFEM, which yielded complementary db values. These db values were neither affected by the nature of the cosurfactant nor by the addition of membrane proteins. These findings illustrate that a biomimetic surfactant sponge phase can be created in the presence of several common membrane protein-solubilizing detergents, thus making it a versatile medium for membrane protein studies.