This book constitutes the proceedings of the 6th International Workshop on Cooperation and Interoperability, Architecture and Ontology (CIAO! 2010), held at the DESRIST 2010 conference in St. Gallen, Switzerland, on June 4, 2010.The 6 papers included in the book were carefully reviewed and selected from 13 submissions. The topics covered are Enterprise Ontology, Organizational Modeling, and System Development.