The influence of pitch precursor composition on the formation and properties of isotropic pitch fibers from non-conventional precursors has been examined. Changes in fiber weight that occur during the oxidative stabilization of green fibers are inversely related to the observed axial contraction. The weight gain upon stabilization also increases, and the contraction decreases with increasing pitch carbon content and aromaticity while the opposite occurs with increasing hydrogen and hetero-atom (H, N, O and S) content. Similar trends are found for fiber carbonization. The combined effects of stabilization and carbonization give a fiber yield of between 50 to 86% of the green fibers, with axial contractions of 12 to 28%, the highest yield corresponding to the smallest contraction. The net yield increases with pitch carbon content and aromaticity, and decreases with hetero-atom content, while the overall axial contraction decreases. The fiber tensile strength was found to increase with precursor carbon content, carbon yield and aromaticity. The activation rate of the derived fibers increased with increasing heteroatom content, especially oxygen content. While most fibers were microporous upon activation, fibers from shale oil and the subbituminous coal extract, developed a more mesoporous structure. (C) 2001 Published by Elsevier Science Ltd.