A method and arrangement for improving hydrolysis of a reduction agent in an exhaust gas post treatment system for selective catalytic reduction of NOx in the exhaust gas of an internal combustion engine operated with excess air. A partial exhaust gas stream is branched off from an exhaust gas stream upstream of an SCR catalytic converter. A hydrolysis catalytic converter is disposed in the partial exhaust gas stream downstream of a metering arrangement for supplying reduction agent thereto. The partial stream is conveyed back to the main stream downstream of the hydrolysis catalytic converter and upstream of the SCR catalytic converter. The temperature at the hydrolysis catalytic converter is maintained within prescribed limits by actively controlling exhaust gas quantity conveyed in the partial stream by entirely or partially blocking the partial stream, via a flow control device, if temperature values at the hydrolysis catalytic converter and the main or partial stream exceed or do not reach specified or threshold values, and/or if certain operating conditions or condition changes of the engine occur.