In this work, the effects of thermal annealing on the structures and properties of an ionic liquid (IL) of 1-butyl-3-methylimidazolium hexafluorophosphate-loaded supercritical carbon dioxide-treated electrospun poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) membrane were studied and compared with a corresponding IL/PVDF-HFP gel membrane. The results show that although the annealing causes slight distortion of the nanofibers as well as some small changes in crystallinity and crystal size, the nanofibrous and microporous morphology of the membrane is not altered and hence the membrane can retain its high optical transparency and ionic conductivity after the annealing, whereas the gel exhibits significantly reduced optical transmittance and ionic conductivity owing to annealing-induced phase separation. The IL/PVDF-HFP electrospun membranes before and after annealing were used as the electrolyte layer in electrochromic devices, respectively. The device with the annealed IL/PVDF-HFP electrospun membrane shows no performance deterioration, demonstrating the potential of such thermally stable electrolyte for electrochromic devices used in harsh environments. (C) 2011 Elsevier B.V. All rights reserved.