The thin film of pentacene on a SiO2 surface has been investigated by infrared spectroscopy in the multiple internal reflections (MIR) mode. It was found that the molecules in the monolayer are arranged with their molecular axes perpendicular to the surface, and that this arrangement is conserved during film growth up to a 70-nm thickness. In addition, the assignment of the infrared active vibrational modes is discussed. (c) 2004 Elsevier B.V. All rights reserved.