Protein hydration plays a crucial role in almost all aspects of biomolecular processes. In this research, we studied the hydration/dehydration- induced infrared amide I band-shift by using poly-L-lysine and bovine pancreas ribonuclease A as model polypeptides. It was found that a 1-4 cm-1 shift could be clearly distinguished for all regular secondary structures during protein thermal unfolding. This shift was proven to be due to backbone hydration but not from experimental error, temperature effect or possible incomplete hydrogen/deuterium exchange of the samples. Moreover, we also found that protein aggregation was closely associated with the backbone hydration/dehydration status of proteins. In conditions favoring aggregation, a significant shift to a higher wavenumber of the band from the intermolecular β-sheet structures in aggregates was observed. The present study suggested that the changes of the amounts of regular secondary structures could be monitored by the intensity changes, while the changes of the hydration status could be monitored by the shift of the infrared bands. © 2008 Bentham Science Publishers Ltd.