Today Information Systems (IS) integration constitutes one of the major success factors of mergers and acquisitions. This article draws on two case studies of firms having realized more than 10 mergers and acquisitions between 1990 and 2000. This paper shows the importance of carrying out a double approach to understand IS integration process. The first approach represents the necessity of using organizational configuration to define possible IS integration modes. Thus we show the importance of organizational, strategic and technological contingencies within the elaboration of integration mode. Then, we complete our analysis with a second approach based on the organizational change theory so as to determine two IS integration process dynamics, i.e. a prescribed integration and a constructed one. These two dynamics allow to apprehend the difficulties in implementing the integration modes chosen for the IS field.