A BROAD absorption feature centred on λ2, 200A with a half-width of ∼ 300A appears in the spectra of reddened stars1–3. This conspicuous feature in the interstellar extinction curve, might hold an important clue to the identity of a major component of interstellar matter, but it has defied identification for over a decade. Here we identify this band as representing the integrated effect of a set of bicyclic compounds, each with the empirical formula C8H6N2. Such nitrogenated structures could form in stellar mass flows of the type which we have also discussed4. A significant mass fraction of all interstellar material might exist in this form.