Relatively little is known about the electrical and environmental stability of the polymeric thin film transistors. Trapping of carriers in active devices can be detrimental to their performance so having a good understanding of the factors that affect it is important. Organic polymers can absorb significant quantities of impurities from vapor relative to the carrier concentration in operating devices. These properties have been used to form vapor sensors with organic TFTs, but have not been extensively from the perspective of the lifetime of operational systems. We will discuss the effects of humidity and oxygen on the device characteristics of TFTs made with polythiophenes such as poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene] PQT-12, and poly(3-hexylthiophene), P3HT. Water strongly increases gate bias stress, but a smaller effect on the field effect mobility of carriers. Exposure to pure oxygen was found to have little effect on the device characteristics of PQT-12 and P3HT over long time periods (months); in contrast, for most polythiophene studied brief exposure to the ambient environment causes a reduction in the on-to-off current ratio. These results can be rationalized from simple chemical principles of reactivity suggesting that these types of studies can aid in the design of new materials and sensors.