This paper describes a query algebra for queries over XML p2p databases that provides explicit mechanisms for modeling data dissemination, replication constraints, and for capturing the transient nature of data and replicas. Categories and Subject Descriptors: H.3.5[Information Storage and Retrieval]: Online Information Service - Data sharing; H.2.5[Database Management]: Heterogeneous Databases; H.2.3[Database Management]: Languages - Data description languages (DDL) General Terms: Algorithms, Management, Languages