In situ diffuse reflectance UV visible and Raman scattering spectroscopies were used to monitor the spontaneous ionization of anthracene, phenanthrene and 9,10-dimethylanthracene by direct exposure to thermally activated acid HZSM-5 zeolite. Calcination of the zeolite under oxygen is a prerequisite for the spontaneous ionization through the formation of Lewis acid sites. The framework structure of ZSM-5 zeolites contains two types of intersecting channels sufficiently wide to allow rod-shaped molecules like anthracene to pass through and to diffuse into the void space. As expected, molecular modeling calculations suggest that anthracene is able to penetrate into the void space of zeolite and that phenanthrene and dimethylanthracene cannot pass through due to steric constraints. The anthracene radical cation is found to be generated in high yield and to be stable over several months. The tight fit between the shape of anthracene and the pore size of the straight channels of the zeolite is considered to be the main factor responsible for the stabilization of the anthracene radical ion. Dimethylanthracene is found to generate dimethylanthracene radical cation in very low yield. Dimethylanthracene radical cation is probably located at the pore openings and is not stabilized in the void space. Ionization of phenanthrene occurs in low yield. However the resulting stable radical cation is identified as anthracene radical cation. The assumption is made that phenanthrene isomerizes readily at the openings of pores via its radical cation.