This paper reports on two sets of data. In the first one, organic thin-film transistors (OTFTs) where fabricated by vapor depositing a pentacene layer on a gate dielectric made of alumina. The devices divide in two sets, depending on whether the alumina surface was or was not modified with a self-assembled monolayer (SAM) of eicosanoic acid. Atomic force microscopy (AFM) images show that the presence of the SAM strongly reduces the size of the crystal grains. A careful analysis of the current-voltage characteristics of the devices, which includes the use of the Transfer Line Method (TLM), allowed the extraction of the gate voltage (VG ) dependent mobility corrected for contact resistance. A remarkable feature is that the mobility decreases with VG at high gate bias, which is interpreted by assuming that the mobility in the layer close to the insulator-semiconductor interface is substantially lower than that in the bulk of the film. Modeling the charge distribution in the conducting channel allowed us to extract the bulk mobility, which is found to saturate at around 5 cm2/Vs in SAM-modified devices and 3 cm2/Vs in devices with bare alumina. The difference is interpreted in terms of more or less defective grains. The second set of results deals with a transistor, in which the active element is reduced to a single monomolecular layer. This was achieved by using a SAM of a bifunctional molecule comprising a quaterthiophene moiety linked to a short alkyl chain. AFM images of the SAM after various adsorption times show that the size of the well-organized domains is around 25 ± 5 nm. Working transistors could only be realized by reducing the channel length L down to a magnitude comparable to that of the domain size, which was achieved with the help of e-beam lithography. In one occasion, well-defined current-voltage characteristics was recorded, which allowed to extract a gate-voltage independent mobility of 0.0035 cm2/Vs.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.