The synthesis of highly microporous, epoxy-functionalized porous organic polymers (ep-POPs) by a one-pot, catalyst-free Diels-Alder cycloaddition polymerization is reported. The high oxygen content of ep-POPs offer efficient hydrogen-bonding sites for water molecules, thus leading to high water-uptake capacities up to 39.2-42.4wt% under a wide temperature range of 5-45 degrees C, which covers the span of climatic conditions and manufacturing applications in which such materials might be used. Importantly, ep-POPs demonstrated regeneration temperatures as low as 55 degrees C, as well as excellent water stability, recyclability, and high specific surface areas up to 852 m (2)g(-1).