The photoinduced ion chemistry of acetonitrile is investigated by using tuneable VUV synchrotron radiation in an octupolar ion guide with mass spectrometric detection. The appearance energies and yields of primary photoions and secondary ionic products deriving from self-reactions at thermal collision energies are measured. Comparison of the appearance energies of the different product ions provides insights into the reaction mechanisms. Experimental data are interpreted with the help of theoretical calculations (energies and structures at the G3 level of theory) for the various isomers of the most relevant ionic products. We find that the ion C2H2N+ is formed at the threshold with a cyclic structure, but only its linear isomer (lying 0.47 eV higher in energy) can react with acetonitrile to form CH2CNCH2+ plus HCN via a cyclic transition state