This book constitutes the thoroughly refereed proceedings of the 5th International Conference on the Quality of Software Architectures, QoSA 2009, held in East Stroudsbury, PA, USA in June 2009, in conjunction with the 12th International Symposium on Component Based Software Engineering (CBSE 2009). The 13 revised full papers were carefully reviewed and selected from 33 submissions. The papers are organized in topical sections on model-driven quality analysis, architectural performance prediction, architectural knowledge, as well as case studies and experience reports.