This study explores the anharmonicity and intermolecular interactions of N-methylformamide (NMF) and di-N,N-methylformamide (DMF) in the neat liquid phase, with a focus on amide bands. We analyzed the vibrational spectra in the mid- (MIR) and near-infrared (NIR) regions (11,500–560 cm−1; 870–17,857 nm) including the complex refractive index. Using Classical Damped Harmonic Oscillator (CDHO) theory, we simultaneously modeled the real and imaginary components of the spectra, identifying band positions, oscillator strengths, and damping constants from the experimental data. This provided insights into vibrational energy dissipation and self-association in liquid amides. The identification of MIR and NIR bands was based on anharmonic GVPT2//B3PLYP/6–311 +  + G(d,p) calculations. The distinct self-association patterns of DMF and NMF were revealed in the MIR fingerprint region by the two components of the νC = O band, analogous to the Amide I band. These findings are further supported by structural information derived from NIR spectra. Additionally, the study examined the contribution of overtones and combination bands in the MIR spectra of amides. Our conclusions on the molecular interactions and structural dynamics of NMF and DMF enhance the understanding of how changes in the local environment affect the amide group.