The interaction of acetonitrile on Pt(111) electrode has been studied using density functional theory methods. Periodic calculations for a coverage of 0.25 show that the most stable adsorption mode of acetonitrile in UHV conditions on Pt(111) is the side-on eta(2) (C; N) state. In this state, the short-bridge surface site is energetically preferred to the long-bridge site. The nitrogen end-on on top of a surface platinum atom is also likely to occur. An external uniform electric field introduced in the cluster models perpendicular to the surface serves to mimic the electrode potential. It makes the adsorbed molecule move and orient perpendicular to the surface. Depending on the direction of the electric field, the acetonitrile is oriented with the nitrogen atom or the methyl group toward the surface. Our results are in agreement with recent SFG experiments performed in electrochemical environments.