The present invention relates to a method and network system for charging an account related to a terminal device (1) of a subscriber (A) to a first data network (10) for a network session rendered to the terminal device when roaming in a second data network (18). According to the method of the invention, upon registration of the terminal device to the second (visited) data network a network address of a first charging system (16) related to the home data network (10) of the terminal device (1) is transmitted from the home data network to the visited data network (18). When a network session is established for the terminal device (1) by the second data network, assessing of charge information about the network session will begin in the second data network in a real-time process. The saved network address of the charging system will be used for transmitting in real-time the assessed charge information from the second data network to the first charging system (16). The first charging system will calculate the charge for the network session using incoming charge information from the second network. The account related to the terminal device (1) will be charged so that the balance of the account is reduced by the charged amount.