Abstract This paper discusses the main parameters to be considered when using Nuclear Magnetic Resonance Spectroscopy for speciation in liquid phase CO2 capture absorbents and how accurate measurements can be obtained. Liquid phase speciation is one of the most important experimental data for thermodynamic and kinetic modeling of absorption systems. A comprehensive standard method for liquid phase speciation in amine and amino acid–CO2–H2O systems by Nuclear Magnetic Resonance Spectroscopy is presented and validated against measurements on monoamine, diamine and amino acid systems. The CO2 loadings calculated from NMR and titration methods are shown to be in good agreement with an AARD less than 1%. The most important factor for a quantitative measurement with 13C NMR is delay time which is required to be ≥5 times the relaxation time (T1) and 300 numbers of scans. The relaxation times (T1) of carbon nucleus of several amines commonly used in CO2 capture, i.e. monoethanolamine (MEA), diethanolamine (DEA), ethylenediamine (EDA), 2-amino-2 methylpropan-1-ol (AMP), N-methyldiethanolamine (MDEA) and piperazine (PZ) as well as for loaded aqueous solutions of MEA–amino acids salt systems of Glycine, Taurine, l-Alanine, l-Serine, l-Proline and Sarcosine are determined and reported in the present work. The relaxation time for the carbon nuclei of HCO3−/CO32− in monoamine systems increases in this order: AMP (3.5s)=DEA (3.5s)<DEEA (8.0s)<MDEA (9.3s)<MEA (10.2s). In diamine systems we have: PZ (4.6s)<EDA (7.7s). The same trend is also observed for carbon nuclei in carbamate formation of monoamine systems: AMP (4.2s)<DEA (5.4s)<MEA (9.7s). Relaxation time for carbon nuclei in diamine dicarbamate formation systems is in this order: PZ (6.5s)<EDA (6.9s). The relaxation times for unloaded systems are not shown because the carbamate and carbonate/bicarbonate species are formed only in the loaded aqueous systems.