The collagen model peptide (Pro-Pro-Gly)₁₀ is known to fold into a triple helix in solution. So far, the triple helix has been considered to exist as a single state. However, our previous study of (Pro-Pro-Gly)₁₀ in solution has indicated the presence of two different states of the triple helix, a lower (H_L) and a higher temperature state (H_H). In the present study, these triple-helical states were investigated in more detail by NMR. Complete stereospecific assignments of the methylene protons of the proline residues were accomplished by the use of NOESY and TOCSY spectra. The temperature dependence of the ¹H chemical shifts showed that the H_L-to-H_H thermal transition can be attributed to a conformational change of the first proline (Pro1) residues of the (Pro-Pro-Gly) triplets. Since TOCSY spectra with a 10 ms mixing-time confirmed a down puckering of these Pro residues in the H_L state, but interconverting down and up puckerings in the H_H state, the H_L-to-H_H thermal transition corresponds to conformational changes of the pyrrolidine rings of the Pro1 residues from an uniform down puckering to a more flexible state. The results confirm that thermal unfolding of the triple helix proceeds through the intermediate H_H state.