In the presence of molecular oxygen and zeolite H-beta with Si/Al-2 = 500, o-xylene reacted regioselectively with liquid nitrogen dioxide at 35 degrees C to yield mononitro-o-xylenes as the main product, where the 4-nitro-o-xylene isomer predominated up to 89% and the 4-nitro-/3-nitro-o-xylene isomer ratio improved to 7.8. The process is eco-friendly, less expensive, and the zeolite could be easily regenerated by a simple workup to afford results similar to those obtained with the fresh catalyst.