Herein we report the synthesis of a new metal–organic framework, Ni(1,4-bib)(inca) 2 or pcu-1-Ni, where 1,4-bib = 1,4-bis(imidazole-1-yl)benzene, inca = indazole-5-carboxylic acid, through the crystal engineering strategy of using an N-donor linker to pillar a square lattice, sql, topology net. pcu-1-Ni adopts pcu topology and features two types of hydrophobic pore, small pore A and large pore B. The biporous nature of pcu-1-Ni is reflected in its stepped CO 2 and H 2 O adsorption isotherms, highlighting the influence of pore size and chemistry on gas and water vapour sorption properties. pcu-1-Ni exhibits the unusual combination of high CO 2 /N 2 selectivity (IAST selectivity 100–250) and low water affinity at low RH (an S-shaped water vapour isotherm with an inflection point at 45–65% RH). Whereas pcu-1-Ni degrades upon repeated exposures to water vapour, its structure–property relationships can provide guidance for design of the next generation of CO 2 -selective sorbents. In this context, Canonical Monte Carlo simulations provide insight into the preferential adsorption of CO 2 over N 2 and H 2 O.