Analyses of extensive experiments with organic thin-film transistors (OTFTs) indicate that the threshold voltage VT of an OTFT has a temporal differential sensitivity. In particular, VT changes initially by changing the light illumination intensity or making/removing a contact of water with the organic semiconductor. Keeping the conditions stationary, then the initial shift of VT diminishes, since the time dependence of VT gradually recovers the OTFT to the state before applying the change in the environmental conditions. While still causing a differential and time-variant shift of VT, the deionized water does not have a dramatic impact on OTFTs that use the polymer DKPP-βT (diketopyrrolopyrrole β-unsubstituted quaterthiophene) as the active semiconductor material. Observations for the impact of water are made from experiments with an OTFT that has a microfluidic channel on the top the electrical channel, with the water in the microfluidic channel in direct contact with the electrical channel of the OTFT. This arrangement of electrical and microfluidic channels is a novel structure of the microfluidic OTFT, suitable for sensing applications of liquid analytes by means of organic electronics.