Abstract The CO and NO stretching modes, in thirteen solvents of varied polarity and polarizability, have been examined in the following complexes of Co and Fe: Co(CO) 3NO, Co(CO) 2NOL, Co(CO)NOL 2, Co(CO)NOL′, Fe(CO) 2(NO) 2, Fe(CO)(NO) 2L, Fe(NO) 2L 2, Fe(NO) 2L′, where L = P (C 6H 5) 3 and L′ = (C 6H 5) 2P-CH 2CH 2-P(C 6H 5) 2. The C 5H 5NiNO complex was used as standard for the nitrosylic bands. Δν/ν correlation plots for the same or for different molecules, show a general linearity and suggest the occurrence of specific vibrator—solvent interactions. The solvent sensitivity of ν (NO) increases continuously both for the Co and Fe complexes on adding phosphine ligands. The antisymmetrical modes are, for the same type of substitution, more sensitive to solvent. Besides, the solvent effect does not always appear to be greater the lower the vibrational frequency. In the carbonyl—nitrosyl complexes the solvent effect on ν (CO) is smaller than on ν (NO) and often negative (i.e., higher frequencies in polar solvents). Such a behaviour gradually returns normal by phosphine substitution. The relative weight of the possible factors of solute—solvent interaction energy justifies qualitatively the observed effects and the direction of the frequency shifts.