A method for controlling a supply of refrigerant to an evaporator (2) of a vapour compression system (1) is disclosed. During a system identification phase an opening degree (12) of the expansion valve (3) is alternatingly increased and decreased, and a maximum temperature difference, (S 4 -S 2 ) max , between temperature, S 4 , of air flowing away from the evaporator (2) and temperature, S 2 , of refrigerant leaving the evaporator (2) is determined. During normal operation, the supply of refrigerant to the evaporator (2) is controlled by calculating a reference temperature, S 2,ref , based on the monitored temperature, S 4 , and the maximum temperature difference, (S 4 -S 2 ) max , determined during the system identification phase. The supply of refrigerant to the evaporator (2) is controlled in order to obtain a temperature, S 2 , of refrigerant leaving the evaporator (2) which is substantially equal to the calculated reference temperature, S 2,ref .