In this article, we report on organic thin film transistors based on a modified oligo-p-phenylevinylene oligomer, 1,4-bis[4-(4-octylphenyl)styryl]-benzene (oligo-S) on chemically modified gate dielectrics. Previously, we have reported on oligo-S devices fabricated on unmodified SiO2 surfaces [T. C. Gorjanc, I. Levesque, C. Py, and M. D'Iorio, Appl. Phys. Lett. (submitted)]. By using either hexamethyldisilazane (HMDS) or octadecyltrichlorosilane (OTS), both the hole mobility and on/off current ratios, I-on/off, have been increased. Maximum mobilities and on/off ratios of 0.14 cm(2)/Vs with I-on/off > 10(6) and 0.26 cm(2)/Vs with I-on/off > 10(7) were achieved with HMDS and OTS modified devices, respectively. (C) 2004 American Vacuum Society.