AbstractUsing high‐resolution Raman spectroscopy, very weak features in the biphonon spectra of disordered materials were investigated. The analysis was focused on the bound states due to the Fermi resonance in crystalline CO2. Two systems were studied: the mixed isotpic (12CO2)x—(13CO2)1−xcrystals and the molecular alloys (N2O)x—(12CO2)1−xand (N2O)x—(13CO2)1−x. In the first system, it was shown that the wavenumbers and bandwidths of biphonon states at K ≈︁ O follow a concentration dependence identical with that encountered for one‐phonon states which obey the separated band limit model in mixed isotopic crystals. In the second system, the occurrence of a fine structure in the biphonon band is discussed on the basis of a strong interaction between non local matrix biphonons and quasi‐local biphonons cr