Exhaust gas aftertreatment systems, which reduce nitrogen oxide emissions of heavy-duty diesel engines, commonly use a selective catalytic reduction (SCR) catalyst. Currently, emissions are controlled by evaluating NOx or NH3 in the gas phase downstream the catalyst and calculating the NH3 loading via a chemical storage model. Here, a microwave-cavity perturbation method is proposed in which electromagnetic waves are excited by probe feeds and the reflected signals are measured. At distinct resonance frequencies, the reflection coefficient shows a pronounced minimum. These resonance frequencies depend almost linearly on the NH3 loading of a zeolite-based SCR catalyst. Since the NH3 loading-dependent electrical properties of the catalyst material itself are measured, the amount of stored ammonia can be determined directly and in situ. The cross-sensitivity towards water can be reduced almost completely by selecting an appropriate frequency range.