Solid polymer electrolytes are widely used in batteries and fuel cells because of the high ionic conductivity that can be achieved at room temperature. The ions are usually Li or protons, although other ions can be shown to conduct in these polymer films. There has been very little published work on solid polymer electrolyte films used as chemical sensors. We have found that thin films of polymers like polyethylene oxide (PEO) are very sensitive to low concentrations of volatile organic compounds (VOCs) such as common solvents. Evidence of a new sensing mechanism involving the percolation of ions through narrow channels of amorphous polymer is presented. We will present impedance spectroscopy of PEO films in the frequency range 0.0001 Hz to 1 MHz for different concentrations of VOCs and relative humidity. We find that the measurement frequency is important for distinguishing ionic conductivity from the double layer capacitance and the parasitic capacitance.