The effect on the secondary structure of representative α-helical, β-sheet and disordered proteins by varying concentrations of dimethyl sulphoxide (DMSO) in 2H2O has been investigated by Fourier transform infrared spectroscopy. Significant perturbations of protein secondary structure are induced by DMSO and DMSO/2H2O mixtures. For highly structured proteins, such as myoglobin and concanavalin A, the infrared spectra point to a progressive destabilisation of the secondary structure until at moderate DMSO concentrations (around 0.33 mol fraction) intermolecular β-sheet formation and aggregation are induced, as indicated by the appearance of a strong band at 1621 cm². This is a direct consequence of the disruption of intramolecular peptide group interactions by DMSO (partial unfolding). At higher DMSO concentrations (above 0.75 mol fraction), such aggregates are dissociated by disruption of the intermolecular C=O…²H-N deuterium bonds. The presence of a single amide I band at 1662 cm⁻¹ corresponding to free amide C=O groups indicates that at high concentrations and in pure DMSO the proteins are completely unfolded, lacking any secondary structure. While low concentrations of DMSO showed no detectable effect upon the gross secondary structure of myoglobin and concanavalin A, the thermal stability of both proteins was markedly reduced. In α-casein, a highly unstructured protein, the situation is one of direct competition. The amide I maximum in 2H2O, at 1645 cm⁻¹, is typical of unordered proteins with C=O groups deuterium-bonded predominantly to ²H₂O. Addition of DMSO disrupts such interactions by competing with the peptide C=O group for the deuterium bond donor capacity of the ²H₂O, and so progressively increases the amide I maximum until it stabilizes at 1663 cm⁻¹, a position indicative of free C=O groups.