A porous polymer network (PPN) grafted with sulfonic acid (PPN-6-SO3H) and its lithium salt (PPN-6-SO3Li) exhibit significant increases in isosteric heats of CO2 adsorption and CO2-uptake capacities. IAST calculations using single-component-isotherm data and a 15/85 CO2/N-2 ratio at 295 K and 1 bar revealed that the sulfonate-grafted PPN-6 networks show exceptionally high adsorption selectivity for CO2 over N-2 (155 and 414 for PPN-6-SO3H and PPN-6-SO3Li, respectively). Since these PPNs also possess ultrahigh physicochemical stability, practical applications in postcombustion capture of CO2 lie well within the realm of possibility.