Mobile devices have access to personal, potentially sensitive data, and there is a growing number of mobile apps that have access to it and often transmit this personally identifiable information (PII) over the network. In this paper, we present an approach for detecting such PII “leaks” in network packets going out of the device, by first monitoring network packets on the device itself and then applying classifiers that can predict with high accuracy whether a packet contains a PII leak and of which type. We evaluate the performance of our classifiers using datasets that we collected and analyzed from scratch. We also report preliminary results that show that collaboration among users can further improve classification accuracy, thus motivating crowdsourcing and/or distributed learning of privacy leaks.