Sedimentation equilibrium in the analytical ultracentrifuge provides membrane biochemists with a tool to quantitatively probe thermodynamics of associating systems in detergent environments. As long as conditions of reversibility are met, the free energy of interaction can be measured in varied hydrophobic environments, pH values, ionic strengths, and temperatures. Although the absolute value of the interaction free energy of membrane protein subunits will no doubt depend on the hydrophobic environment, experiments in any one environment will allow subunit associations to be placed on a relative scale of interaction. The temperature dependence of the free energy change may provide more thorough information about the thermodynamics of helix-helix association in micelles.