An experimental study reveals that plasma polymers containing high concentrations of stable radicals and π-conjugated moieties are powerful probe materials for molecular sensing. Specific and sensitive sensor systems can be constructed of quartz crystals coated with these films, and molecular classification techniques can be used to analyze the transient piezoelectric responses of these crystals. Sensor probes consisting of plasma polymer films coated on quartz crystal microbalance are investigated. These sensor systems respond reversibly and reproducibly, alternating exposure to vaporized molecules and to air. Molecular classification mapping, reflecting physiochemical characteristics of target molecules, gives valuable information for molecular sensing.