Poly-L-proline has been used as a model system for various purposes over a period of more than 60 years. Its relevance among the protein/peptide community stems from its use as a reference system for determining the conformational distributions of unfolded peptides and proteins, its use as a molecular ruler, and from the pivotal role of proline residues in conformational transitions and protein-protein interactions. While several studies indicate that polyproline can aggregate and precipitate in aqueous solution, a systematic study of the aggregation process is still outstanding. We found, by means of UV-circular dichroism and IR measurements, that polyproline is predominantly monomeric at room temperature at millimolar concentrations. Upon heating, the polypeptide stays in its monomeric state until the temperature reaches a threshold of ca. 60 degrees C. At higher temperatures, the peptide aggregates as a film on the inside surface of the employed cuvette. The process proceeds on a time scale of 10(3) s and can best be described by a bi-exponential relaxation function. The respective CD and IR spectra are qualitatively different from the canonical spectra of polyproline in aqueous solution, and are indicative of a highly packed state.