A system is provided for determining a parameter indicative of an amount of a reducing agent in a selective catalytic reduction (SCR) module of a vehicle, the system including: the SCR module having at least one passage for transporting a stream of gas and an SCR catalyst for converting NOx emissions, and having an extension in an axial direction, at least one first antenna unit configured to transmit and receive a microwave signal. Moreover, the first antenna unit is arranged outside the SCR module and configured to transmit microwave signals towards the SCR catalyst and to receive a microwave signal reflected within the SCR module. The system further includes processing circuitry connected to the first antenna unit and configured to determine a difference between the transmitted microwave signal and the received reflected microwave signal and to determine a parameter indicative of the amount of a reducing agent in the SCR catalyst based on the determined difference between the transmitted microwave signal and the received reflected microwave signal. Moreover, the invention relates to a method for determining a parameter indicative of an amount of a reducing agent in a system of a vehicle.