Thin films of iodine-doped polyphenylacetylene (PPA) have been investigated and used as humidity sensors. Measurements of resistance variations of a PPA/I2 system, induced by relative humidity variations ranging from 10 to 90%, have been performed at constant temperatures (25 and 30-degrees-C). The behaviour of the resistance versus relative humidity is found to be approximately exponential. Our samples show a good reversibility during humidity cycles and a relatively fast response time (a few seconds).