A number of studies show that chemical modification of the semiconductor–dielectric interface can be used to control the threshold voltage (Vth) of organic thin film transistors (OTFTs). A promising chemical functionality to achieve that are acidic groups, which – at the semiconductor–dielectric interface – have been used to realize chemically responsive OTFTs and easy to fabricate inverter structures. Especially for pentacene based OTFTs, the underlying chemical and physical mechanisms behind the acid-induced Vth shifts are not yet fully understood. Their clarification is the topic of the present paper. To distinguish between space-charge and dipole-induced effects, we study the impact of the thickness of the gate oxide on the device characteristics achieving maximum Vth-shifts around 100 V. To elucidate the role of the acid, we compare the doping of pentacene by acidic interfacial layers with the impact of hydrochloric acid vapour and investigate the consequences of exposing devices to ammonia. Complementary experiments using 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) as active layer hint toward the central (6 and 13) carbon atoms being subject to the electrophilic attack by the acidic protons. They also prove that the observed Vth shifts in pentacene devices are indeed a consequence of the interaction between the acidic groups and the active material. The experimental device characterization is supported by drift-diffusion based device modelling, by quantum chemically simulations, as well as by contact angle, atomic force microscopy (AFM) and X-ray reflectivity (XRR). The combination of the obtained results leads us to suggest proton transfer doping at the semiconductor–dielectric interface as the process responsible for the observed shift of Vth.