Carbon molecular sieve (CMS) membranes are usually prepared from dense polymeric precursors that already show intrinsic gas separation properties. The rationale behind this approach is that the occurrence of any kind of initial porosity will deteriorate the final CMS performance. We will show that it is not necessary to produce a non-porous precursor in order to obtain a selective CMS membrane. We used tight ultra-filtration (UF) fiber membranes as a precursor. These fibers did not have any gas separation properties before the pyrolysis treatment, nor were coatings applied to these fibers before or subsequent to the pyrolysis. After a heat treatment in air followed by a pyrolysis in a nitrogen atmosphere CMS fiber membranes were obtained. The CMS fibers were analyzed using scanning electron microscopy, thermo gravimetrical analysis, and gas permeation. From the permeation rates and permselectivity values measured for He, H2, CO2, Ar, O2, N2, CH4, C2H4, C2H6, C3H6, C3H8 and SF6 the evolution of the mean pore diameter was investigated. It was found that the pore diameter increases with pyrolysis temperature up to 800 °C, but decreases as the temperature is raised to 900 °C. The overall porosity reaches its highest value at 900 °C.