A microporous three-dimensional hydrogen-bonded organic framework (HOF-5) has been constructed from a new organic linker 4,4',4 '',4'''-tetra(2,4-diamino-1,3,5-triazin-6-yptetraphenylethene. Activated HOF-5a exhibits a stepwise N-2 adsorption isotherm at 77 K, suggesting framework flexibility. The structure of activated HOF-5a has been established by powder X-ray diffraction studies, indicating a significant framework contraction from as-synthesized HOF-5 to activated HOF-5a of similar to 21% by volume. HOF-5a shows moderately high porosity with a Brunauer-Emmett-Teller (BET) surface area of 1101 m(2)/g, and takes up a large amount of acetylene and carbon dioxide under ambient conditions. Powder neutron diffraction studies and theoretical calculations reveal that suitable pore sizes, curvatures, and functional sites collectively enable HOF-5a to encapsulate a high density of carbon dioxide molecules packed in a pseudo-one-dimensional array along the pore channel.