Electrochemical potential induced orientation changes of polar molecules at the quartz-CH3CN interface have been studied using sum-frequency generation spectroscopy. Our result demonstrates the dramatic tuning effect of the electrochemical potential on the orientation of molecules in the electrical double-layer region. For D289(+) cations with a large dipole moment (similar to 14D), the average molecular orientation is found to change from 90 degrees to about 34 degrees when the electrochemical potential is changed from the potential of zero charge to -0.9 V. From the orientation data, the electrical field experienced by molecules in the electrical double-layer region and the effective thickness of the electrical double layer have been extracted : E approximate to 5 x 10(6) V/cm and d approximate to 20 Angstrom, respectively.