The work presented here reports an investigation into the role that the semicon-ductor-insulator interface may play in the threshold voltage instability that has been observed in polymer devices. For this purpose, metal-insulator-semiconduc-tor capacitors and field-effect transistors have been fabricated and studied by means of admittance spectroscopy and field-effect characterization. Poly(3-hexyl-thiophene) has been used as the organic semiconductor whereas polyimide and polysilsesquioxane films have been used as the gate insulator. The capacitance and loss as a function of frequency for different voltages and temperatures presented different dispersions that were analyzed in terms of different equivalent circuits where the presence of bulk states, interface states, traps in the insulator and interfacial dipoles were independently taken into account. In this manner, it was found t hat t he interface between poly(3-hexylthiophene) and polysilsesquioxane presented two different sets of interface states at different energies within the band gap. The shallower set was characterized by having a density of interface states of~ 1011 -1013 cm-2ev-1 while the deeper set had a density of states ~ 1010 -1011 cm-2ev-1 and a larger capture cross section. While the presence of the shallower set of stat es seemed to have little or no effect on the observed device instability, charging the deeper set, by applying a large positive gate voltage, caused a positive threshold voltage shift. In the devices where polyimide was used as t he gate insulator, the admittance data was consistent with an interfacial dipole of density ~ 5 • 109 dipoles/cm2. The formation of such dipoles resulted in a positive threshold voltage shift upon a pplication of large positive gate volt ages. The field-effect transistors studied for t his work suffered from high subthreshold slopes due to the relatively thick P3HT film so t hat the subthreshold regime was dominated by current flowing through t he undepleted portion of the film rather than by interfacial effects. Nevertheless, similar threshold voltage instabilities were found from admittance and field-effect transistor measurements suggesting that the same mechanism (int erface states in polysilsesquioxane-based devices and interface dipoles in polyimide-based devices) is responsible in both cases.