For the first time, heterogeneous catalytic reactions have been monitored by in situ EPR spectroscopy in the Q-band using a homemade heatable probe head equipped with a flow reactor. The reactions of Al2O3-supported TEMPO with NO and H-2 as well as of SiO2/Al2O3-supported H4PVMo11O40 with methanol and formaldehyde were studied up to 400 degrees C. TEMPO radicals are immobilized on the support in positions which impose a different reactivity to NO and H-2. This may be due to different accessibility, which changes during thermal treatment. By combined evaluation of anisotropic X- and Q-band spectra with a complex hyperfine structure (e.g., from VO2+), spin Hamiltonian parameters can be derived with higher precision, since limits of the specific resolution in both frequency bands are compensated for. In addition to VO2+, Mo5+ is formed above 180 degrees C depending on the O-2 content of the feed, which is easily discriminated in the Q-band but not in the X-band.