Electron paramagnetic resonance (EPR) can be collected both under ex situ, in situ, and operando conditions on solid catalyst materials exposed to relevant gas mixtures. The reactions can be followed with a time resolution of less than a minute and the EPR response can be used to quantify the content of the EPR active component. This is exemplified by the reaction between NO, NH3, and O2 to form N2 and water, the selective catalytic reduction (SCR) reaction. Reactive SCR catalysts containing redox active copper and vanadium find use in industrial applications. Both metals alternate between two oxidation states, one diamagnetic and one paramagnetic having an informative EPR spectrum. The spectral resolution at the relevant temperatures (150–250 °C) in in situ or operando experiments is often compromised and the main information obtained is the change in spectrum intensity with time and reaction conditions. Here we present and compare the information obtained on a copper-exchanged zeolite material and on a vanadium-substituted polyoxometalate immobilized on titania during in situ cycling between different gas mixtures relevant for the SCR reaction.