A two-part study utilizing isotactic poly(4-methyl-l-pentene) (PMP) was undertaken to investigate a three-stage process (melt-extrusion/annealing/uniaxial stretching) (MEAUS) utilized to produce microporous films. In this report, the thermal-annealing (second stage) and subsequent uniaxial-stretching (third stage) results of selected PMP films from three resins, labeled A, B, and C, are discussed. From sequential analysis of the effect each stage had on the resulting microporosity, it was discovered that the melt-extruded precursor morphology and orientation, as a consequence of the first-stage extrusion parameters and resin characteristics, were crucial to controlling the membrane permeability. The annealing parameters were also critical, where a temperature of 205degreesC applied for 20 min under no tension was the optimum annealing condition for producing highly microporous PMP films upon stretching. For the conditions studied, the stretching parameters that were found to be the optimum for producing the desired characteristics in the final film were cold- and hot-stretch temperatures of 70 and 180degreesC, respectively. The cold- and hot-stretch extension levels concluded to be the best were a cold-stretch extension of 80%, followed by hot stretching to 90%, and, thus, a total overall extension level of 170% for the processing window studied. However, these results were only with respect to resin A films, while resin B and C samples could not be produced into microporous films via the MEAUS process.