Isomeric trimethylbenzyl radicals were generated and vibronically excited from precursor 1,2,3,5-tetramethylbenzene, isodurene, with a large amount inert carrier gas helium in a corona excited supersonic expansion (CESE) using a pinhole-type glass nozzle. A long-path monochromator was used to record the visible vibronic emission spectra of the jet-cooled benzyl-type radicals in the $D_1{ ightarrow}D_0$ electronic transition. From the analysis of the spectra, we identified the evidence of the presence of three isomeric trimethylbenzyl radicals in the corona discharge, and obtained the electronic energy and a few vibrational mode frequencies in the ground electronic state for each isomer.