Acid-soap crystals of stoichiometry NaH(2)A(3) (A = the RCO(2)(-) moiety of the C-12 through C-18 fatty acids) have been isolated in a pure state by crystallization from aqueous solutions and characterized by X-ray diffraction. The sodium palmitate-palmitic acid acid-soap NaH2P3 was characterized, in addition, using FT-IR, calorimetry, and solid-state C-13 NMR spectroscopy. The chains in the acid-soap crystals are tilted at 32.7 degrees relative to the normal of the lamellar sheets and display a perpendicular subcell packing. Both NMR and infrared data indicate that two distinct carbonyl carbons exist. These differ from the carbonyl carbons in pure fatty acid and soap crystals, although one is still carboxylic acid-like in structure, while the other is carboxylate ion-like in structure. These acid-soap crystals are anhydrous and do not form crystal hydrates. NaH2P3 crystals are modified above 70.0 degrees C to form a polymorph showing a distinctly different X-ray diffraction pattern. These new data require revision of the McBain sodium palmitate-palmitic acid phase diagram.