The modification of insulating surfaces by self-assembled monolayers (SAMs) is an elegant way of tailoring the gate dielectric of organic field effect transistors (OFET) to pentacene and is commonly used to improve electrical performance. A SAM based on an alkylcarboxylic acid deposited on a thin layer of alumina, serving as the gate dielectric is considered. The relationship between carrier mobility and (i) the length of the carboxylic acid (CH3(CH2)(n)COOH; n = 9, 14, 18), (ii) substrate preparation and (iii) the SAM and pentacene thin film structures is considered. The size and boundaries of pentacene grains are not limiting factors for carrier mobility, and the most relevant parameter, which depends on whether there is a SAM or not, is the organization of the first pentacene layers in contact with the gate dielectric. The variation of the interplanar distance d(001) of the pentacene layers close to the alumina surface is much greater without SAM than with, and this could explain the lower carrier mobility observed in the case of a bare alumina dielectric. The relationship between the disorder associated with this variation and mobility is discussed. (C) 2015 Elsevier B.V. All rights reserved.