ESCRT-I is required for the sorting of integral membrane proteins to the lysosome, or vacuole in yeast, for cytokinesis in animal cells, and for the budding of HIV-1 from human macrophages and T lymphocytes. ESCRT-I is a heterotetramer of Vps23, Vps28, Vps37, and Mvb12. The crystal structures of the core complex and the ubiquitin E2 variant and Vps28 C-terminal domains have been determined, but internal flexibility has prevented crystallization of intact ESCRT-I. Here we have characterized the structure of ESCRT-I in solution by simultaneous structural refinement against small-angle X-ray scattering and double electron-electron resonance spectroscopy of spin-labeled complexes. An ensemble of at least six structures, comprising an equally populated mixture of closed and open conformations, was necessary to fit all of the data. This structural ensemble was cross-validated against single-molecule FRET spectroscopy, which suggested the presence of a continuum of open states. ESCRT-I in solution thus appears to consist of an approximately 50% population of one or a few related closed conformations, with the other 50% populating a continuum of open conformations. These conformations provide reference points for the structural pathway by which ESCRT-I induces membrane buds.