Sedimentation equilibrium experiments in the Beckman Optima XL-A Analytical Ultracentrifuge (Fullerton, CA) can be used to determine aggregation states of synthetic peptides in homogeneous associating systems such as in which P is peptide monomer and K = [P_n/[P]ⁿ. Experimental conditions are chosen so that transport by sedimentation is balanced by diffusional transport within a sample. The equilibrium concentration distribution formed over a period of several hours is analyzed using known monomeric molecular weights to determine the aggregation number and thermodynamic parameters such as equilibrium constants, ΔG, ΔH, ΔS, and ΔCp. It is preferable to use three different concentrations of the same peptide, run simultaneously, and to achieve equilibria at several different temperatures.