We report a study on the photoionization of the C₆H₆ isomer 3,4-dimethylenecyclobutene, DMCB. The molecule is an intermediate in the formation of benzene from the propargyl radical self-reaction, a suggested first step in the formation of polycyclic aromatic hydrocarbons and soot. From a threshold photoelectron spectrum we determine an adiabatic ionization energy of 8.75 eV. The geometry change upon ionization is associated with considerable vibrational activity, which is assigned to a symmetric in-plane bending mode of the =CH₂ groups. A breakdown diagram shows that the dissociative photoionization resembles the one observed for benzene. Computations reveal that DMCB cation isomerizes to the benzene cation and dissociates from there.