The quantitative characterization of reversible protein-protein interactions is fundamental to the elucidation of basic biological function as well as the development of new biotherapeutics.  Although mass spectrometry, array-based assays and computational analysis have been crucial for the prediction of protein interactomes, such high-throughput methodologies fall somewhat short in characterizing protein interactions, for example providing binding constants, discerning between affinity and avidity, etc.  We present here a direct method for post-screen characterization of interaction partners.  Composition Gradient Multi-Angle Light Scattering (CG-MALS) determines stoichiometry and equilibrium association constants of self- and hetero-associations. This is accomplished without recourse to sample modifications such as fluorescent tagging or surface immobilization, and with no restrictions on buffer composition – the experiment may be carried out in native solution or the desired formulation buffer.