Apparatus for generating an ammonia-containing gas is described for use in the selective catalytic reduction of NOx contained in the exhaust gases of an IC engine. The apparatus has a hydrolysis reactor (101) for containing an aqueous solution of urea that is heated, in use, to an elevated temperature by way of heat exchange with the exhaust gases to hydrolyse the urea and liberate ammonia containing gases. A pressure control valve (105) is operable between a substantially closed position for enabling the pressure of the ammonia-containing gas to attain a predetermined elevated pressure within the reactor (101) and an open position when the gas is above the predetermined pressure. A reservoir (106) receives all of the ammonia-containing gas discharged from the reactor (101) when the pressure control valve (101) is in its open position and has an outlet for feeding ammonia-containing gas to the exhaust gases. The reservoir (106) stores ammonia-containing gas during operation of the IC engine and, following the IC engine being switched off, ammonia-containing gas condensate. On cold start-up of the IC engine, the ammonia-containing gas condensate is converted into ammonia-containing gas for feeding to the exhaust gases.