Irreversible protein–protein aggregation is associated with several human maladies, including Alzheimer's disease, and poses a challenge to pharmaceutical formulation and preservation efforts. The existence of multiple agglomeration pathways that lead to different final morphologies complicates the ability to determine the aggregation fate of proteins under various reaction conditions. Using lysozyme (3 mg mL−1) as a model protein for dynamic light scattering and circular dichroism studies, we observed dense, non-fibrous aggregation triggered under acidic (pH 4) conditions by a destabilizing “ethanol shock” of 16% (v/v) ethanol. However,ethanol concentrations 15% and below did not lead to aggregation. The aggregation fate of these proteins is thus found to be extremely sensitive to the denaturant concentration. We introduce a stochastic model to explain this dependence based on the denaturant-induced population of a partially unfolded state.