Molecular surface doping was studied for organic thin film transistors consisting of an active layer of few monolayers pentacene, as prepared by physical vapor deposition. In situ transport measurements with different dopants revealed both positive (tetrafluoro-tetracyanoquinodimethane - F₄TCNQ) and negative (manganese(III)-tetraphenylporphyrin-chloride), as well as zero value (cobalt(II)-tetraphenylporphyrin and fullerene) gate threshold shifts. For F₄TCNQ, a high doping efficiency of 25 % was observed. The maximum gate threshold shift was more than halved with pentacene thickness increasing from 2.5 to 5 monolayers, indicating that the doping effect decays above ∼2.5 monolayers. Charge transfer has been discussed based on complementary X-ray photoelectron experiments.