We show that the electrochemically grown aluminum oxide dielectric films always comprise some redox-active molecular species absorbed from the electrolyte. This contamination affects dramatically electrical performance of organic field-effect transistors (OFETs), particularly leading to the appearance of hysteresis in the current-voltage characteristics and positive shift of the threshold voltage. A strong suppression of the hysteresis was observed while reducing the concentration of the citric acid or replacing it with an alternative electrolyte based on the aminoacid (isoleucine) and its potassium salt.