Vapor-phase doping of poly(p-phenylene vinylene) (PPV) with fuming sulfuric acid has been investigated using the quartz crystal microbalance. Thin films of PPV were obtained on quartz crystal substrates by spin casting a precursor polymer, which is thermally converted to give the PPV product. Dopant mass uptake and the resistance of the film were recorded in sitar during doping. A decrease in the resistance and therefore an increase in the conductivity of the film from the insulating to the metallic regime was observed. Mass uptake information was used to determine the dopant concentration at the maximum conductivity, the doping rate, and the diffusion coefficient of the polymer. The dopant concentration results were verified by Rutherford backscattering spectrometry and elemental analysis. The observation that the doped material tends to lose its metallic properties upon exposure to moisture in the atmospheric air has been used to investigate its potential as a humidity-sensing device. The golden metallic appearance of the film changes as well. The conductivity can be returned by application of a dynamic vacuum. Fuming sulfuric acid-doped films were exposed to a range of relative humidities, froth 4 to 75%. A rapid and significant linear response (over four orders of magnitude) to all percent humidities was observed. Fuming sulfuric acid-doped PPV may have the potential for applications as a humidity sensor or as a disposable shut-off switch in moisture-sensitive devices.