Semi-interpenetrating polymer networks (s-IPNs) fabricated from linear polyimides entwined by crosslinked poly(ethylene oxide) (PEO) networks are reported in this work as CO2-selective separation membranes. While keeping the overall PEO content constant (similar to 70 wt%), the structural characteristics of s-IPNs, i.e., the distribution and the connectivity of PEO domains, were systematically varied by adjusting the PEO content in the linear polyimide component and the crosslink density of the PEO networks, leading to six unique s-IPNs. Tested s-IPNs showed significantly improved mechanical strength and ductility relative to crosslinked pure PEO membranes, with the tensile strength of s-IPNs being nearly 4-times and > 7-times as high as those of the crosslinked pure PEO membranes with high and low crosslink density, respectively. Moreover, all s-IPNs featured greatly improved CO2 permeability (up to 25 fold) and CO2/H-2 and CO2/N-2 selectivity compared to their linear (co) polyimide counterparts. Several s-IPNs outperform the 2008 Robeson upper bound for CO2/H-2 and closely approach the CO2/N-2 upper bound. In general, incorporating PEO segments in the linear polyimides and utilizing low-crosslink-density PEO networks are beneficial in improving both CO2 permeability and ideal selectivity. Overall these PEO-rich s-IPNs provide an attractive membrane platform for high performance CO2 separation applications.