Dynamic properties are very useful in the specification of Information Systems (IS) and security policies. They allow the user to express properties that involve several states of a system. Indeed, invariance properties do not permit to cover such kind of properties. In this paper, we suggest a formal approach, based on the use of the B method, to verifying absence properties of the form Abs(P2, From P1 Until P3) that express that some states, represented by predicate P2, should not be reached starting from a state that satisfies P1 until a state satisfies P3 is reached. Our proposal consists in defining two proof obligations based on weakest preconditions that are sufficient and necessary to prove that a system verifies such a property.