Implications regarding requirements of performing a successive electrospinning and producing nanoporous polyetherimide (PEI) fibers are discussed through electrospinning PEI solutions of three nonvolatile solvents, that is, dimethylforamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP), under atmospheres of constant temperature and varying levels of relative humidity (RH). The results demonstrate depending on nature of miscibility area in ternary phase diagram, a minimum RH is necessary to stabilize fiber formation. Furthermore, RH of operating environment affects diameter and both surface and interior morphologies of PEI electrospun fibers through involving the rate of phase demixing and viscoelasticity of solution. Considering fibers produced from NMP solutions because of delayed demixing, solvent drying precedes phase demixing or takes place in a comparable rate in high RH which leads to solid cross-section and texture-less surface with slight porosity. By choosing DMF or DMAc as electrospinning solvent, thicker fibers with rough surface and porous cross-section are expected.