The continuous, flexible, and self-supporting nanocomposite films of EuW10/PAN (EuW10 = Na-9[EuW10O36]center dot 32H(2)O; PAN = polyacrylonitrile) have been successfully fabricated through electrospinning technique, and the resulting films display strong red emission of Eu(III) ion. When the films are exposed to acidic gases such as HCl, H2S and SO2, the red luminescence can be quenched. Upon further exposure to NH3 gas, the nanocomposite films show the recovered red luminescence. As such, the EuW10/PAN nanocomposite films exhibit reversible, high-contrast, and pH-modulated luminescent photoswitching, which involves two distinct states that can be interconverted between the protonation and deprotonation states of EuW10. This work could be of benefit for the design and fabrication of novel electro-optical devices.