A series of thermo‐sensitive blending films was prepared from hydroxypropylcellulose (HPC) and polyacrylonitrile (PAN). The effects of materials ratio, pH value, and temperature on the swelling velocity of these blending films were studied. At temperatures above the lower critical solution temperature (LCST), different results are found after dipping blend film samples in acid or alkaline solutions, respectively. At a pH value of 1.4, the swelling velocity of HPC/PAN blended films increased with the HPC content. The films' swelling was mainly controlled by polymer chain relaxation. For a pH value of 7.4 the mechanism responsible for the swelling is the water molecules' diffusion. The swelling velocity was also affected by temperature and pH. Copyright © 2009 John Wiley & Sons, Ltd.