Photodissociation of acetonitrile (CH3CN) condensed, physisorbed on Au(111), and chemisorbed on Pt(111), has been studied at the C(1s) and N(1s) edges in H+ yield and/or in total positive ion yield. The H+ yield C(1s) absorption experiment on the condensed acetonitrile shows a selective fragmentation of the C-H bond at the C 1s -->pi*CH3 excitation, that is still observed when acetonitrile is physisorbed on Au(111) and chemisorbed on Pt(111). Transitions to the pi*C=N state give rise to unselective fragmentations, which are strongly quenched by charge transfer from the substrate when acetonitrile is chemisorbed on Pt(111). The pi(perpendicular to)*C=N level that plunges into the Pt(111) is the most affected and vanishes in the total ion yield measurement. The quenching is weaker on the pi(parallel to)*C=N which is parallel to the surface, and thus is less hybridized with the metal than the pi(perpendicular to)*C=N. For acetonitrile physisorbed on Au(111), the influence of the surface is limited to the quenching of the desorption of the heaviest outgoing fragments.