Studies show that the physical implementation of an interorganizational business process or system can be a major source of operational problems and reduced business benefits. Better process modeling has been advocated as a solution. Although powerful modeling tools exist, current practice often gives short-shrift to documenting the physical implementation details that can create or exacerbate such problems. In this paper we describe the modeling approach we devised for the interorganizational business processes and systems we observe in our ongoing fieldwork. Our approach involves using allowable extensions to a popular modeling notation (BPMN), although other modeling tools would work equally well. We illustrate the benefit of our approach in the case of the Internet Payment Platform, a pilot project of the United States Department of the Treasury.