Two different organic molecules with similar structure, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-dimethyl-3,4,9, 10-perylenetetracarboxylic diimide (DiMe-PTCDI), were used for the modification of Ag Schottky contacts on sulphur passivated GaAs(I 0 0) (S-GaAs). Such diodes were investigated recording in situ current-voltage (I-V) characteristics. As a function of the PTCDA thickness the effective barrier height of Ag/PTCDA/S-GaAs contacts initially increases from 0.59 +/- 0.01 to 0.72 +/- 0.01 eV, and then decreases to 0.54 +/- 0.01 eV, while only a decrease in barrier height from 0.54 +/- 0.01 to 0.45 +/- 0.01 eV is observed for DiMe-PTCDI interlayers. The initial increase and decrease in effective barrier height for PTCDA and DiMe-PTCDI respectively, is correlated with the energy level alignment of the lowest unoccupied molecular orbital (LUMO) with respect to the conduction band minimum (CBM) of S-GaAs at the organic/inorganic semiconductor interface. Whilst there is an additional barrier for electrons at the PTCDA/S-GaAs interface of about 150 meV, i.e. the LUMO lies above CBM, the LUMO is aligned or below CBM in the DiMe-PTCDI case. The results also shine light on the important issue of the transport gap in organic semiconductors for which an estimation can be obtained. (C) 2003 Elsevier Science B.V. All rights reserved.