Abstract: In this paper, the Fourier infrared spectra of N,N-dimethylformamide-(DMF ) and N,N-dimethylacetamide (DMA) and their water-containing sample have been investigated in the range 100-4000cm^-1. The changes of frequencies and intensities resulted by the hydration of DMFand DMA　have been observed from the substraction spectra.The bands at 1674 and 1647cm^-1 are assigned to the C＝O　stretching vibration for the DMF　and DMA, which decrease to 1651 and 1626cm^-1 respectively due to the formation of hydrogen bond between C＝O　and water upon hydration.The hydration of DMF　and DMA　may also be characterized by the formation of hydrogen bond with the lone electron pair of the amide nitrogen. This may be seen from the substraction spectra of water-containing DMF　and DMA　in the IRfinger-print region and also in the far IRregion. The following bands change upon hydration. 1389, 1094, 659, 404, 354 and 329cm^-1 for DMF; 1397, 1188, 1014 and 591cm^-1 for DMA.On the other hand, the characteristic frequencies of pure water peaks at 1646 and 3403cm^-1 also change to 1690 and 3500cm^-1 upon hydration with DMF, or to 1663 and 3469cm^-1 upon hydration with DMA, because the electron density of the water molecule is increased as a result of receiving some electronic charge from the Lewis base.