Organic thin-film transistor (OTFT) performance depends on the chemical characteristics of the interface between functional semiconductor/dielectric/conductor materials. Here we report for the first time that OTFT response in top-gate architectures strongly depends on the substrate chemical functionalization. Depending on the nature of the substrate surface, dramatic variations and opposite trends of the TFT threshold voltage (similar to +/- 50 V) and OFF current (10(5) x !) are observed for both p- and n-channel semiconductors. However, the field-effect mobility varies only marginally (similar to 2 x). Our results demonstrate that the substrate is not a mere passive mechanical support.