Polyacrylonitrile (PAN) porous fibers were prepared in one step by electrospinning a ternary system of PAN/N, N-dimethylformamide (DMF)/water at ambient environment. The formation of porous structures was mainly due to the spinodal decomposition phase separation occurred during the electrospinning process The concentration of PAN varied from 3 to 10 wt % with the water content changing from 2 to 8 wt % When PAN concentration was above 5 wt %, the composition of the ternary system was close to the "cloud point" and fibers with porous structures were obtained In addition, the surface tension and viscosity of PAN solutions increased with water which may lead to the diameter increase of the fibers. The Brunauer-Emmett-Teller (BET) surface area of porous PAN nanofibers obtained from 8 wt.% PAN solution containing 7 wt % of water was 46.4 m(2)g(-1), which was 3 times higher than that of nonporous PAN nanofibers prepared under the same conditions from a solution without water. (C) 2010 Elsevier B.V. All rights reserved