The transition states for prenucleation assembly, nucleation, and growth of aggregates and amyloid fibrils were investigated for a dimeric immunoglobulin light chain variable domain, employing pressure, temperature, and solutes as variables. Pressure-induced aggregation was nucleation-dependent and first-order in protein concentration and could be seeded. The insoluble aggregates were mixtures of amyloid fibrils and amorphous aggregates. Activation volumes, activation surface areas, and activation waters of hydration were larger for aggregate growth than for prenucleation assembly or nucleation, although activation free energies were similar for the three processes. Activation free energies for each of the transition states were dominated by the unfavorable free energy of solvation of newly exposed surfaces. Equilibrium dissociation and unfolding of the dimer showed a much larger volume change than those required to form the transition states for the three processes. Thus, the transition states for these steps are similar to the native state, and their formation requires only small structural perturbations. Finally, the presence of Congo red during amyloid fibril formation shortened lag times and caused pressure insensitivity of nucleation, suggesting that this compound or its analogs may not be effective as inhibitors of amyloidosis.