In summary, this work shows that O NMR is an excellent probe of different CO 2 adsorption products in amine functionalised adsorbents. In particular, O NMR can differentiate between ammonium carbamate chains and carbamic acids in a wide range of materials. Our measurements provide new support for ammonium carbamate chain formation in a series of (amine)-Mg 2 (dobpdc) variants, and also provide strong evidence for a recently proposed mixed ammonium carbamate -carbamic acid mechanism for the material (dmpn)-Mg 2 (dobpdc). We reveal carbamic acid formation in a previously poorly studied adsorbent, (ii-2)-Mg 2 (dobpdc), highlighting the prevalence of carbamic acid in frameworks with bulky amine groups. Finally, initial measurements on amine-grafted silica materials showcase the excellent versatility of the technique, and support the formation of ammonium carbamates in these materials, while also suggesting a new adsorption mode may be in operation. It is worth noting that great care was taken to ensure no water was present during the preparation of samples. It is known that the presence of water would impact the CO 2 adsorption mechanism and O NMR spectroscopy would be a powerful tool to explore additional mechanisms further. In the future O NMR spectroscopy will be extended to a range of carbon capture technologies and will ultimately enable the design of improved materials that can help tackle the climate crisis.