Résumé : The X+2Πu ← X1Σ+g transition of dicyanoacetylene has been recorded for the first time using pulsed-field-ionisation zero-kinetic-energy photoelectron spectroscopy. The analysis of the photoelectron spectrum allowed an accurate determination of the adiabatic ionisation potential of C4N2 (Ei, ad./hc = 95,479 ± 2 cm-1) and a description of the vibrational structure of the electronic ground state of the cation which is affected by the Renner-Teller effect and the spin-orbit interaction. The spin-orbit coupling constant was measured as -52 ± 2 cm-1. These results are supported by ab initio calculations performed at the complete active space self-consistent field and second-order perturbation theory levels of theory, with extrapolation to the complete basis set limit.