We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect transistors based on polyphenylenevinylene derivatives. Through a systematic investigation of the most common dielectric surface treatments, a direct correlation of their effect on the field-effect electrical parameters, such as charge carrier mobility, On/Off current ratio, threshold voltage, and current hysteresis, has been established. It is found that the presence of OH groups at the dielectric surface, already known to act as carrier traps for electrons, decreases the hole mobility whereas it does not substantially affect the other electrical characteristics. The treatment of silicon dioxide surfaces with gas phase molecules such as octadecyltrichlorosilane and hexamethyldisilazane leads to an improvement in hole mobility as well as to a decrease in current hysteresis. The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the semiconductor polymer can be related not only to the OH groups density but also to the interaction between the dielectric and the semiconductor molecules. Specifically, the elimination of the OH groups produces the same effect observed with hexamethyldisilazane. The hole mobility values obtained with hexamethyldisilazane and polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.