In this study, we performed differential scanning calorimetry (DSC) and pressure perturbation calorimetry (PPC) analysis of the thermal transition of cytochrome c from an acidic molten globule (MG) state with the protein concentrations of 0.5-18.2 mg/mL. DSC profiles were highly reversible and showed clear protein-concentration dependence, indicating that reversible oligomerization occurred accompanying the thermal transition from the MG state. The DSC and PPC data required at least a six-state model (MG1 ⇄ MG2 ⇄ D ⇄ 1/2 I2 ⇄ 1/3 I3 ⇄ 1/4 I4) including three new oligomeric states: dimer (I2), trimer (I3), and tetramer (I4) in addition to the three monomeric states previously characterized. Dynamic light scattering confirmed the oligomerization during the thermal transition. The partial specific volumes of these oligomeric states were found to be smaller than those of the monomeric states, MG2 and D, indicating dehydration of hydrophobic surface or hydration of released anions may occur with the reversible oligomerization.