A system and method for allowing users to specify using commonly available mobile devices such as mobile telephones, transactions to be performed is described. More particularly, the invention provides a transaction interface layer which uses a predetermined lightweight message syntax to allow a user to specify transactions to be performed. The particular transactions envisaged are financial transactions such as stock or share trades, but the invention also allows other transaction types, such as information transactions, to be specified. When an information transaction is specified, the system of the invention acts to filter received information in accordance with a user-specified transaction, and to forward filtered information to a user mobile device as appropriate.