The present invention relates to a method and a system for initiating and/or conducting a transaction that is associated with at least two corresponding declarations of intent, in particular a payment transaction, between at least two transaction parties via a processing unit (3), in which payment transaction at least one of the transaction parties uses a land line telephone or mobile telephone (1, 2) or a mobile communications device for transmitting data. The method is characterized by the transmission of data to the processing unit by at least two transaction parties within a limited time frame, and by the fact that the initiation of the data transmission of these transaction parties is effected actively by the transaction parties and not by the processing unit (3), while the data transmitted during initiation contain features that enable correlating the declarations of intent with one another.