Protected amino acids and dipeptides of the type Boc-X-NHCH 3 and Boc-X-Y-NHCH 3 (where X=Pro, Val, Y=Thr and Boc=t-butyloxycarbonyl) have been investigated by infrared spectroscopy in solution phase. The NH stretching and CO stretching regions of the spectra were studied by means of Fourier self-deconvolution and curve fitting to determine the number, frequency and intensity of band components. The results were interpreted by taking into account the effects of all possible intramolecular hydrogen bonds, including a perturbation effect of a C=O H-N bond on the CO group adjacent to the NH.