|The long term bias stress stability of poly(quaterthiophene) thin film transistors is described. Transistors are subject to repetitive pulsed gate bias stress for periods of up to five months, and the threshold voltage and mobility are monitored. The samples are unencapsulated bottom gate transistors held in either clean dry air or dry nitrogen. The threshold voltage shift increases as a power law with time for the first 1-2 days, and the magnitude of the shift is a strong function of gate voltage. After a few days, threshold voltage shift stabilizes to an approximately constant value of 2-10 V for gate voltages of 20-40 V. When the stress is removed, the threshold voltage recovers with a time constant of 2-4 days, and the recovery is the origin of the stabilization. The gate bias stress also induces a reduction in mobility of 10%-30% over about ten days, and there are further slow changes in mobility, which may be unrelated to electrical stress. (c) 2006 American Institute of Physics.