A method of controlling the value of at least one on-line transaction involving a token associated with an authorized user comprises the steps of: prior to the at least one proposed transaction, receiving at a pre-transaction authorization station, pre-transaction authorization data. The authorization data comprises first time data relating to a time window within which the at least one proposed transaction must take place and first amount data relating to an amount with which a value of the at least one proposed transaction must comply. Substantially at the time of a proposed transaction, transaction data comprising second time data relating to a time instant of the proposed transaction and second amount data relating to a value of the proposed transaction is received at the authorization station. The authentication data and transaction data are compared in real time at 34 and only if the transaction data complies with the authorization data, is the transaction enabled.