Pore size and pore size distribution of nanofibrous membranes have been studied using capillary flow porometry. Nanofibrous membranes have been obtained by electrospinning polymer solutions either at room temperature or at elevated temperature. Our results reveal that pore size and pore size distribution are strongly associated with fiber mass, fiber diameter, and fiber length. An increase in membrane mass caused a decrease in pore size and a shift in pore size distribution towards lower values. Larger fiber diameter resulted in the formation of larger pores. The dependence of pore size on fiber length has also been discussed and a correlation is proposed between the pore size of electrospun membranes and fiber length. The current study is significant in providing a general Understanding of the porous structure of electrospun fibrous membranes because the pore size and distribution of the membranes depend on the physical size and amount of fibers and are independent of the composition of the spun materials. (c) 2006 Elsevier B.V. All rights reserved.