<p>Hydrogen bonds’ flexible distances and moderate strength entitle compounds to dynamic properties under external stimuli. Here we report multiple phase transitions and counter-intuitive CO₂ adsorption behavior of dynamic guanidinium sulfate (GS) salt assembled via hydrogen-bonds. Exploration based on the energy landscape generated by crystal structure prediction (CSP) reveals three porous GS phases with stability of α &gt; β &gt; γ and the inverse order of porosity, agreeing with experimental results. Transformations among polymorphs via heating or compressing involve ion rearrangement. Adsorption isotherms of β-GS indicate that CO₂ firstly enters the isolated cavities at a low gating pressure, and further increasing CO₂ pressure leads to the continuous gas uptake but reduced pressure at a critical point and thus an unexpected negative pressure inflexion (NPI), followed by the final adsorption saturation. Theoretical calculations demonstrate that the NPI behavior stemmed from the GS structural transition from β to more porous γ-phase, with the γ-GS phase becoming more energy-favorable as CO₂ uptake increases. Specific supramolecular interactions ensure CO₂ selectivity and easy regeneration. With a CO₂ uptake of 4.2 mmol g^-1 (273 K, 100 kPa), GS salt exhibits great promise for CO₂ capture and transport, demonstrating the potential of simple hydrogen-bonded salts as adaptive materials.</p>