The bile salts, as natural steroidal surfactants, are different from common surfactants and have a rigid steroid backbone, with unusual facially molecular structure. The bile salts play divergent roles in biology and in supramolecular chemistry owing to their hierarchical crystal structure of associated helical assemblies (1). Here we report on the formation of oppositely charged, so-called "catanionic" (2) aqueous surfactant mixtures of an anionic sodium cholate with the positively charged n-alkylammonium chlorides, by varying the surfactant chain length (the number of carbon atoms per chain is 10, 12, 14 and 16). In these amphiphilic mixtures surfactants react electrostatically and strong synergism is found (3). The formation of giant needle-like colorless precipitates is observed at higher concentrations, which are identified as exactly 1:1 and 1:2 complex of alkylammonium with cholate molecules, and their new crystal structures are solved. By varying pH, because the cholate headgroup has a carboxylic acid functionality, the morphology of precipitate and the contribution of complexes are continuously changed. The alkylammonium cholates form the molecular cavities by changing the alkyl parts of the ammonium cations. All of the complex crystals belong to the monoclinic space group P21. Comparison of the molecular packings shows that the molecular assembly pattern and the related structural changes vary among different complexes. Unit cell of 1:1 complexes built four (Z = 4), and that of 1:2 complex six molecules (Z = 6). The increase of alkyl chain length results with increase of a and c cell lengths, and cell angle β . The b cell length is constant (approximately 7.9 Å ) and characteristic for this group of investigated compounds, independently also on Z. The 1:1 complexes of decyl-, tetradecyl-, and hexadecylammonium cholates structures have similar crystal packings: the molecules of cholic acid (CA) form hydrogen bonds with each other thus creating characteristic layers with hydrophobic parts on the outside. Between those layers the long aliphatic parts of the alkylammonium are trapped. The charged NH3+ parts form various hydrogen bonds with OH and COO- groups of the CA molecules and with crystal water in the hydrophilic interior. The 1:2 complex of the dodecyl- compound reveals quite different crystal packing: the hydrophilic regions are separated into layers which are not parallel and therefore cross each other thus forming hydrophobic channels which host the alkylammonium cations. References 1. K. Inoue, K. Kato, N. Tohnai, M. Miyata, Published online in Wiley InterScience (www.interscience.wiley.com), 9 June 2004, 4648. 2. N. Filipović-Vinceković, I. Pucić, S. Popović, V. Tomašić, Đ. Težak, J. Colloid Interface Sci., 1997, 188:396. 3. D. Varade, V. Patel, A. Bahadur, P. Bahadur, M.S. Vethamuthu, Indian J. Biochem. Biophys., 2004, 41:107.