This book constitutes the refereed proceedings of the 9th International Symposium on Privacy Enhancing Technologies, PETS 2009, held in Seattle, WA, USA, in August 2009. The 14 revised full papers presented were carefully reviewed and selected from 44 initial submissions. The papers - both from academia and industry - cover design and realization of privacy services for the internet and other communication networks and present novel research on all theoretical and practical aspects of privacy technologies, as well as experimental studies of fielded systems.