Sodium soap fibers with varying alkyl chain lengths were studied by cryotransmission electron microscopy and differential scanning calorimetery in water and water-propylene glycol mixtures. The morphology of the lamellar fibers was found to be dependent on the chain length of the alkyl chain and the solvent polarity. Cryoelectron microscopy revealed that short-chain (C-10-C-14) sodium soaps have the bilayer plane perpendicular to the fiber width, which enables one to see the bilayer striations on the fibers, whereas long-chain (C-16-C-20) sodium soaps have bilayer planes parallel to the fiber width, and the bilayer striations are not visible. This change in morphology is accompanied by a change in dissolution enthalpy.