The features of several adsorption modes of acetonitrile on three selected faces of nickel have been investigated by means of extended Huckel calculations on finite-size clusters. The present results are consistent with the available experimental data on this system. The face for the stablest adsorption is the open (110) one, on which CH3CN preferentially adsorbs parallel to the surface with a binding energy greater than 30 kcal . mol(-1). On the (100) face, acetonitrile is less strongly bound in the same eta(2) geometry. Lastly, on the (111) face, there is a competition between two geometries : one parallel to the surface and the other perpendicular. The underlying electronic effects have been thoroughly analyzed and explained by the balance between stabilizing two-electron interactions and repulsive four-electron ones.