Raman spectra of liquid mixtures of HCONH2 and (HCONH2)-C-13, and of HCOND2 and DCOND2 are presented. The isotropic Raman spectra of the former mixture show two bands in the carbonyl stretching region, whereas the latter only show one. The same phenomenon is observed in the solid mixture at -20 degrees C. At low concentrations of this mixture in DMSO solutions, the bands due to individual molecular species of HCOND2 and DCOND2 are observed. The phenomenon is due to an intermolecular coupling causing oscillators with not too different frequencies to oscillate with the same frequency, if the strength of the coupling exceeds a critical value, K-c. In liquid mixtures of HCONH2 and DCONH2, the isotropic Raman carbonyl bands are to close in frequency to decide whether they show this phenomenon as well. In the case of mixtures of HCONH2 and (HCONH2)-C-13, the intermolecular coupling strength is less than K-c, and the two carbonyl bands due to individual molecular species are observed.