Thermal unfolding, reversible self-association, and irreversible aggregation were investigated for an alanine-rich helical polypeptide, 17-H-6, with sequence [AAAQEAAAAQAAAQAEAAQAAQ](6). Dynamic light scattering, transmission electron microscopy, and thermal unfolding measurements indicate that 17-H-6 spontaneously and reversibly self-associates at acidic pH and low temperature. The resulting multimers have a compact, globular morphology with an average hydrodynamic radius similar to 10-20 nm and reversibly dissociate to monomers upon an increase to pH 7.4. Both free monomer and 17-H-6 chains within the multimers are alpha-helical and folded at low temperature. Reversible unfolding of the monomer occurs upon heating of solutions at pH 7.4. At pH 2.3, heating first causes incomplete dissociation and unfolding of the constituent chains. Further incubation at elevated temperature induces additional structural and morphological changes and results in fibrils with a beta-sheet 2 degrees structure and a characteristic diameter of 5-10 nm (7 nm mean). The ability to modulate association and aggregation suggests opportunities for this class of polypeptides in nanotechnology and biomedical applications.