Electrospun polymers have many applications in the industry. However, the structure of these polymers has been less widely considered by researchers. In this work, the structural effect of electrospun and casted films of poly-acrylonitrile (PAN) and polyvinylidene fluoride (PVDF) polymers on water vapor transmission were investigated. Sorption of water vapor was measured at 35, 60 and 80 degrees C and different relative humidities. The diffusion coefficients were calculated based on mass changes of the polymer sample. The water vapor transmission rate (WWII) was also measured at 35 degrees C and 90% relative humidity. The results indicated that electrospun nano-fibrous polymers (ESNPs) absorb much higher water vapor compared to non-porous casted polymers. The interaction of water molecules with mentioned polymers was investigated based on Flory-Huggins theory. The Flory-Huggins interaction parameter of electrospun films was less than casted films, suggesting much better interaction of water molecules with electrospun films. It was also found that electrospun films have anomalous kinetic behavior and do not obey the Fickian diffusion model. Finally, it was revealed that ESNPs show less resistance to water vapor transmission and they are good candidates for the applications of water vapor separation using membranes. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.