Let ϕ be a faithful normal semi-finite weight on a von Neumann algebra$M$. For each normal semi-finite weight ϕ on$M$, invariant under the modular automorphism group Σ of ϕ, there is a unique self-adjoint positive operator$h$, affiliated with the sub-algebra of fixed-points for Σ, such that ϕ=ϕ($h$·). Conversely, each such$h$determines a Σ-invariant normal semi-finite weight. An easy application of this non-commutative Radon-Nikodym theorem yields the result that$M$is semi-finite if and only if Σ consists of inner automorphisms.