We investigated the morphological, structural and transport properties of pentacene thin films (20 nm) grown by high vacuum thermal evaporation on patterned substrates with different interelectrodes distances. X-ray diffraction and atomic force microscopy show a structure in the so called "thin film phase" with 1-2 mu m sized grains. By means of in situ electrical measurements we found for these films a resistivity value of rho = 4.7 +/- 0.2 x 10(4) Omega m, that is an order of magnitude lower than the value reported to date in literature for single crystals of pentacene. This value is not affected by the presence of grain boundaries. The resistivity is further reduced by a factor 8.9 +/- 0.7, 14 +/- 1, 2.3 +/- 0.3 upon exposure to oxygen, nitrogen and ambient air, respectively. (C) 2006 Elsevier B.V. All rights reserved.