Protein aggregation, leading to the formation of amyloid fibrils, is associated with many human diseases, including Parkinson's disease, Alzheimer's disease and type II diabetes. 2,2,2-trifluoroethanol (TFE) is frequently used to induce amyloid conversion in biophysical studies, but the mechanisms underlying TFE-induced fibrillization are not yet well understood. We have measured secondary structural changes of the Parkinson's disease-associated protein [alpha]-synuclein ([alpha]S), and have discovered that TFE-induced aggregation is correlated with population of a partially structured state of the monomer protein. By investigating the pH- and temperaturedependences of the conformational transitions, we find evidence that loss of proteinsolvent interactions drives both the structural changes and the fibril production. Furthermore, we used enhanced green fluorescent protein (EGFP) as a model system to examine the effects of sequence and tertiary structure in TFE-induced aggregation, and found that the behavior of acid-denatured EGFP is qualitatively similar to [alpha]S, while tertiary structure impedes aggregation. We conclude that initiation of protein aggregation in solutions containing TFE involves overcoming multiple protective factors, rather than stabilization of specific structural elements. We identify three distinct structural states that contribute to the circular dichroism spectra of [alpha]S variants and acid-denatured EGFP. For both types of proteins, a partially [alpha]-helical conformation is populated at moderate TFE concentrations where aggregation is enhanced. The TFE-induced [alpha]S fibrils are [beta]-sheet-rich, flexible, helical structures, while the EGFP aggregates are flexible, uniform-width fibrils. At low (<10-15% v/v) TFE, the [alpha]S variants and acid-denatured EGFP undergo loss of polyproline-II structure, which is suggestive of reduced protein-water interactions. At higher TFE, preferential solvation leads to TFE coating of the proteins, stabilizing [alpha]-helical structures. The temperature response of [alpha]S reveals distinct behavior for proteins in water-like vs. TFE-like local environments. Moreover, the intermediate-TFE conformations appear to be invariant with respect to temperature and pH, which indicates that the proteins experience reduced solvent interactions at moderate [TFE]. Our results suggest that TFE reduces solvation barriers in aggregation reactions. However, aggregation pathway selection may depend on details of protein structure, and the protein sequence affects the TFE concentrations required for dehydration-driven fibrillization.