A new material, polyethynylfluorenol (PEFl), has been prepared and characterized, either undoped or homogeneously doped with SnCl2. We have chosen as basic device for the sensor a simple humistor (humidity sensor resistor) since we have observed that the PEFl conductivity varies with the environmental relative humidity (r.h.). This behaviour is completely reversible without any noticeable 'memory effect'. PEFl can be considered a promising material for the fabrication of humidity sensors.