We report a new semicontinuous process for the production of porous polyetherimide monofilaments. Dense, carbon-dioxide-saturated fibers are spun at rates up to 1 m/s, and porosity is introduced at the spinning head, which establishes the transition from the pressure cell to the heating bath. The process is designed to control independently the carbon dioxide saturation level of the swollen fiber and the temperature during expansion. This subtle control of process conditions allows for the production of closed microcellular as well as open nanoporous monofilaments. The microstructure of the porous monofilaments obtained in our process can be tailored on the basis of a detailed "foam diagram" obtained previously in a discontinuous characterization method. The foam diagram identifies closed microcellular and open nanoporous regions in a temperature versus concentration map.