Various types of sensors to monitor humidity and gases were prepared using organic polymers. In order to prepare resistive type humidity sensors which can be used at high humidities, several methods of cross-linking of hydrophilic polymer chains were proposed, while in order to prepare capacitive humidity sensors durable in organic solvent vapors, utilization of cross-linked hydrophobic polymers were proposed. In addition, for a resistive type $CO_2$ sensors, solid polyethylene glycol containing alkali carbonate can be used. Polymers which have dimethyl amino branch can be used for the sorbing layer in a QCM type sensor. These polymers are alos applicable to the fabrication of both resistive and QCM type acetic acid vapor sensor. Polypyrrole and polyaniline can be used to detect $NH_3$gas.