Abstract: The infrared absorption spectra of the CO monomer isolated in solid N₂ have been recorded at various temperatures between 4.5 and 30 K. The absorption features of the fundamen-tal stretching mode show its linewidth and matrix-induced frequency shift to be weakly temperature-dependent. As the temperature of the matrix was raised, an increase in the linewidth together with a redshift in the central frequency was observed. These observa-tions were explained in terms of the quenching of the CO rotational states by the N₂ matrix into closely-lying librational states. A quantitative model was then used to calculate the energy difference between these librational states. Results show that they can be thermally populated through the absorption of matrix phonons.