For MFU-4l, our results supported a previously proposed bicarbonate capture mode, and a two aspect fast-intermediate exchange regime was identified involving carbonyl 'seesaw' and proton swivelling motions. KHCO 3 -CD-MOF required a multi-faceted analysis approach, including: experimental O NMR comparison to model solid compounds KHCO 3 and K 2 CO 3 .1.5H 2 O with quantitative MLFF-MD modelling, 2D MQMAS and multi-field data, and qualitative MLFF simulations. These combined to give a newly proposed mixed carbonate-bicarbonate mechanism, 1 : 1.6 -1.9 ratio, for KHCO 3 -CD-MOF. Overall, this work shows the potential of O NMR spectroscopy to reveal carbon dioxide speciation and dynamics, and thereby to act as a guide for the design and development of improved hydroxide-based CO 2 capture materials.