Long-standing research efforts have enabled the widespread introduction of organic field-effect transistors (OFETs) in next-generation technologies. Concurrently, environmental and operational stability is the major bottleneck in commercializing OFETs. The underpinning mechanism behind these instabilities is still elusive. Here we demonstrate the effect of ambient air on the performance of p-type polymer field-effect transistors. After exposure to ambient air, the device showed significant variations in performance parameters for around 30 days, and then relatively stable behaviour was observed. Two competing mechanisms influencing environmental stability are the diffusion of moisture and oxygen in the metal–organic interface and the active organic layer of the OFET. We measured the time-dependent contact and channel resistances to probe which mechanism is dominant. We found that the dominant role in the degradation of the device stability is the channel resistance rather than the contact resistance. Through time-dependent Fourier transform infrared (FTIR) analysis, we systematically prove that moisture and oxygen cause performance variation in OFETs. FTIR spectra revealed that water and oxygen interact with the polymer chain and perturb its conjugation, thus resulting in degraded performance of the device upon prolonged exposure to ambient air. Our results are important in addressing the environmental instability of organic devices.