This volume includes 20 contributions of the 12th meeting on Analytical Ultracentrifugation from March 1-2, 2001 in Duisburg, Germany. Various fields of ultracentrifugation are covered concerning research problems in biochemistry, biophysical chemistry and macromolecular chemistry as well as interacting systems. New investigations concerning the sedimentation theory are presented. The phase transition of gels is dealt with, as is the sedimentation-diffusion equilibrium of gels. One section contains the hydrodynamics of biopolymers.Technical and Methodological Investigations.- W. M?chtle, M.D. Lechner: Evaluation of equilibrium and non-equilibrium density gradients in AUC by Calibration with marker particles;G. Lucas, L. B?rger, H. C?lfen: Solubility equilibrium gradients in the Analytical Ultracentrifuge: An approach towards the isolation of critical nuclei in solution;Polymers, Colloids and Supramolecular Systems.- P. Lavrenko, K.Yevlampiiev, D. Volokhova: Hybrid polymer heterogenicity and boundary profile in a low speed sedimentation experiment;C. Tzatzios, H. Durchschlag, C. Eschbaumer, C.H. Weidl, U.S. Schubert, P. Schuck, W. Maechtle, and D. Schubert: Ultracentrifugation studies on a small supramolecular compound. The v problem;V. Vogel, K. Langer, S. Balthasar, P. Schuck, W. M?chtle, W. Haase, J.A. van den Broeck, C. Tzatzios, and D. Schubert: Characterization of serum albumin nanoparticles by sedimentation velocity analysis and electron microscopy;I. Zaitseva, N.A. Michailova, N.P. Yevlampieva, E.I. Rjumtsev, V.S. Khotimsky, E.G. Litvinova, M.V. Chirkova, G.M. Pavlov: Properties of a membrane forming polymer, poly(l-trimethylgermyl-l-propyne) in dilute solutions;Biolocial Systems.- Z. Bozoky, L. F?l?p, G. Janoki: Combination of Preparative and Analytical Ultracentrifugation of the low density lipoprotein (LDL) aliquots to prepare radioactive labelling lipoproteins;G. Berth, H. Dautzenberg; H. C?lfen: Physico-chemical and chemical characterisation of chitosan l“¦