In this work we present a study concerning the influence of some of the most important external factors on the electrical properties of transparent thin-film transistors (TFTs), using zinc oxide produced at room temperature as the semiconductor material. Electrical characterization performed sixteen months after the production of the devices showed a decrease in the on/off ratio from 8×105 to 1×105, mainly due to the increase of the off-current. We also observed a small increase in the saturation mobility, a decrease in the threshold voltage and an increase in the gate voltage swing (by factors of about 1.2, 0.9 and 1.6, respectively). Exposure to ambient light does not have a noticeable effect on the electrical properties, which is an important point as regards the application of these devices in active matrix displays. Some variation of the electrical properties was only detectable under intense white light radiation. In order to evaluate the temperature effect on the TFTs, they were also characterized at 90 ∘C. At this temperature we noticed that the off-current increased more than two times, and the other electrical properties had a small variation, returning to their initial values after cooling, meaning that the process is totally reversible.