This study investigated the temporal appearance of CO₂-derived signals and qualitative spectral differences between rich and lean phases in a diethylenetriamine (DETA)/dimethyl sulfoxide (DMSO)/H₂O (DTSO) biphasic absorbent using FTIR, time-series and phase-separated ¹³C NMR, and DFT calculations. CO₂ reacted mainly with DETA, forming carbamate-related species. The earliest detectable CO₂-derived carbonyl signal appeared at 10 min and was tentatively assigned to a terminal mono-carbamate-related environment. Additional overlapping resonances attributable to candidate carbamate environments appeared as absorption progressed. Phase-separated ¹³C NMR showed carbonyl signals in both phases. Preliminary internal-standard estimates indicated a higher amount associated with the selected DETA-related signal in the analyzed rich-phase aliquot and a slightly higher amount associated with the selected DMSO-related signal in the lean-phase aliquot. However, these estimates do not establish a complete mass balance or quantitative partition coefficients. At the original B3LYP/SMD(H₂O) level, terminal-site candidates had lower calculated relative Gibbs free energies than corresponding central-site candidates. Representative ion-associated structures exhibited N–H···O contacts geometrically consistent with hydrogen bonding, although the relative electronic-energy ordering of the ion-pair models was sensitive to D3(BJ) dispersion treatment. Therefore, the calculations provide a model-dependent comparison of candidate-structure stability, not definitive predictions of solution-phase species populations or ion-pair formation.