The role of the substrate in influencing the structure and morphology of alpha,omega-dihexyl-quaterthiophene (DH4T) thin films grown by organic molecular beam deposition was investigated by means of atomic force microscopy (AFM) imaging and optical absorption measurements. Silica and potassium acid phthalate (KAP) substrates were selected as representative of inorganic amorphous and organic crystalline substrates, respectively. The film growth mechanism was found to depend on the flatness, cleanliness, and structure of the substrate, which influence the overgrown film. (C) 2002 Elsevier Science B.V. All rights reserved.