A method for operating a fluid supply system , said fluid supply system comprising a fluid tank , pumping means and a fluid dosing valve for dosing a fluid into an exhaust gas system of an engine, said method comprising operating said fluid supply system by a plurality of dosing cycles (Tc, T'C) within a pre-determined dosing period (TP), wherein each dosing cycle comprises: determining a desired mass value ( mDes ) of a mass of a fluid to be dosed into the exhaust gas system of the engine, wherein said desired mass value ( mDes ) is determined based on a set mass value ( m Set) of an accumulated required mass of said fluid during a current dosing period (TP), and on a dosed mass value ( mDos) of an accumulated mass of said fluid having been dosed into said exhaust gas system during said cur-rent dosing period (TP) up to and including the previous dosing cycle (Tc), operating said fluid supply system in order to pump said fluid from said fluid tank to said fluid dosing valve and to dose said fluid , via said fluid dosing valve , into said exhaust gas system , and updating said dosed mass value ( mDos ) by accumulating a mass of fluid ( m'Dos ) dosed into the exhaust gas system in the current dosing cycle (T'C) and the dosed mass value ( mDos ) of said accumulated mass of said fluid having been dosed into said exhaust gas system during said cur-rent dosing period (TP) up to and including said previous dosing cycle (Tc) further comprising operating said fluid supply system by a plurality of consecutive ones of said predetermined dosing periods (Tp) wherein, for a first dosing cycle of a current dosing period, said desired mass value ( mDes ) is determined based on a set mass value ( m Set) of an accumulated required mass of said fluid during said current dosing period, on a dosed mass value ( mDos ) of an accumulated mass of said fluid having been dosed into said exhaust gas system during a previous dosing period up to and including a last dosing cycle of said previous dosing period, and on a reset mass value ( m Reset) defining said dosing periods. determining a desired mass value ( mDes ) of a mass of a fluid to be dosed into the exhaust gas system of the engine, wherein said desired mass value ( mDes ) is determined based on a set mass value ( m Set) of an accumulated required mass of said fluid during a current dosing period (TP), and on a dosed mass value ( mDos) of an accumulated mass of said fluid having been dosed into said exhaust gas system during said cur-rent dosing period (TP) up to and including the previous dosing cycle (Tc), operating said fluid supply system in order to pump said fluid from said fluid tank to said fluid dosing valve and to dose said fluid , via said fluid dosing valve , into said exhaust gas system , and updating said dosed mass value ( mDos ) by accumulating a mass of fluid ( m'Dos ) dosed into the exhaust gas system in the current dosing cycle (T'C) and the dosed mass value ( mDos ) of said accumulated mass of said fluid having been dosed into said exhaust gas system during said cur-rent dosing period (TP) up to and including said previous dosing cycle (Tc) further comprising operating said fluid supply system by a plurality of consecutive ones of said predetermined dosing periods (Tp) wherein, for a first dosing cycle of a current dosing period, said desired mass value ( mDes ) is determined based on a set mass value ( m Set) of an accumulated required mass of said fluid during said current dosing period, on a dosed mass value ( mDos ) of an accumulated mass of said fluid having been dosed into said exhaust gas system during a previous dosing period up to and including a last dosing cycle of said previous dosing period, and on a reset mass value ( m Reset) defining said dosing periods. <EOS>