This paper proposes an approach to prove interference freedom for a reachability property of the form AG ψ ⇒ EF φ in a B specification. Such properties frequently occur in security policies and information systems. Reachability is proved by constructing using stepwise algorithmic refinement an abstract program that refines AG ψ ⇒ EF φ. We propose proof obligations to show non-interference, ie, to prove that other operations can be executed in interleave with this program while preserving the reachability property, to cater for the multi-user aspect of information systems. Proof obligations are discharged using conventional B provers (eg , Atelier B). Since refinement preserves these reachability properties, proofs can be conducted on abstract machines rather than implementation code.