A novel quartz resonator was developed to measure, simultaneously, changes in the mass and electrical resistance of a polyaniline film during the absorption of water vapor. Interdigitated gold electrodes were vacuum-deposited on the sensing surfaces of the quartz crystals, and polyaniline films were drop-cast on the electrodes used to measure the changes in the electrical resistance. Two symmetric semicircular gold electrodes were deposited on the bottom surface of the quartz crystal. These electrodes were used to measure the changes in the mass of absorbed water based on the changes in the resonance frequency. The simultaneous measurements of mass and electrical resistance shed light on the interactions between the water vapor and the polyaniline film. The resonator was exposed to various organic gases, including ethanol, acetone, or chloroform, and each gas was found to produce characteristic changes in the normalized electrical resistance.