This paper describes a technique to support interoperable query processing when multiple heterogeneous knowledge servers are accessed. The problem is to support query transformation transparently, so a user can pose queries locally, without any need of global knowledge about diierent data models and schema. In a companion paper, an architecture for supporting interoperability was described and we provided some details of transforming queries from an object schema to an equivalent relational schema. In this paper, we focus on transforming SQL source queries, posed against a rela-tional schema, to XSQL queries to be evaluated against equivalent object schema. We describe some functional modules in detail, namely an extractor module (EM) which extracts semantics from a source query, and a heterogeneous mapping module (HTM) which maps among entities in diierent schema, based on some mapping rules which reeect global knowledge of the models, schema and query languages. The local and global dictionary knowledge is represented in a canonical form, using a high-level logic representation, namely F-logic.