The paper proposes a rigorous approach to the prediction of electromagnetic interaction between tags of an RFID system powered by the incident field radiated by the reader.The design of a 2.45 GHz band passive transponder and its integrated antenna is first developed making use of a numerical optimization based on EM analysis coupled with the harmonic-balance technique. A couple of tags, located at varying mutual positions, is then analysed under digitally modulated RF drive. The results show a significant modification of the electrical and radiation performance of each tag and suggest the need for a multi-tag design procedure to account for these coupling effects.