Building on transaction cost theory, this research analyzes the different forms of B2B integration with regard to their impact on connectivity and coordination costs. Based on a field study from the automotive industry, it demonstrates that there is economic rationale for preferring supplier portals to machine-to-machine integration based on EDIFACT or XML messages. Compared to prior technologies, SOA reduces the costs of external integration by eliminating separate B2B integration infrastructures and improving connectivity of internal applications.