Absorptions simultaneously involving the vibrations of two molecules are brought out of the near infrared spectra of mixtures containing a proton donor RXH and an acceptor R′AB. The donors under study are: chloroform, water, pentachlorophenol, and methanol, and the acceptors: acetonitrile, acetone, dimethylsulfoxide, and pyridine; for comparison, carbon disulfide and hexachloracetone are also used as solvents. Not only the combinations vXH + vAB and vXH + v3(CS2) are observed, but also vXH + vR′, at about 6500 cm−1, if R′ contains CH bonds; these absorptions at 6500 cm−1 do not exist when fully deuterated bases R′AB are used. A correlation between the intensity increase, through hydrogen bonding, of the vXH bands and that of the simultaneous transitions is suggested by the results. In the assignment of the near infrared spectra of proton donors, it is important to take into account the simultaneous transitions because their intensity is sometimes greater than that of the overtones 2vXH. It is likely that these conclusions, founded on the study of relatively weak hydrogen bonds, may be extended to stronger interactions; but the width of the absorption bands in the case of strong hydrogen bond makes the observation of the simultaneous transitions difficult.