We present here a detailed study of the growth process of quaterthiophene (4T) based thin films, especially the influence of end-substitution of alkyl chains on the film formation. A radical change in the growth mechanism introduced by the side chains could be observed by atomic force microscopy (AFM) measurements. While the unsubstituted quaterthiophene shows a typical island growth, an almost perfect layer-to-layer growth was found for the alpha,omega-dihexylquaterthiophene (DH4T). The alteration in the growth mode leads to differences of one order in the grain size going from 4T to DH4T, which results finally in an enhancement of the mobility measured in the field-effect transistor (FET) of the same order. The results reveal a change of the growth process induced by molecular engineering leading to an improvement of the film morphology towards larger grain sizes and thus to an enhancement of the electronic properties of the organic semiconductor thin films. (C) 2003 Elsevier Science B.V. All rights reserved.