We estimate ΔΓ for the B s− B s system by making a sum over exclusive decay modes into ground state mesons, and compare our result to the parton model prediction. For the modes D s D s, D s D ∗ s + D ∗ s D s and D ∗ s D ∗ s (class I) which give the largest contribution to ΔΓ, we find duality between the exclusive calculation and the parton model in the heavy quark limit with 2 m c → m b and with the standard factorization assumption, which amounts to exact duality in the N c → ∞ limit. In this limit, for the dominant spectator contribution, we find the selection rule Γ[B 2(CP=−)→D s D ∗ s]=0 , so that the modes D s D ∗ s + D ∗ s D s contribute constructively to ΔΓ, with the same sign as D s D s (S wave) and D ∗ s D ∗ s ((S + D)-waves). Most of the modes of the modes of class II (ψη, η cϕ, ψϕ, …) contribute also constructively to ΔΓ but are suppressed by the effective color factor. On the other hand, from a purely theoretical point of view, we find a rather subtle and interesting difficulty as regards duality in the standard factorization assumption when considering together classes I and II. This unwanted violation of duality could correspond to the well known necessity of correcting this standard factorization assumption. On the basis of both the exclusive and the parton model calculations, we conclude that the sign of ΔΓ is well-defined and its magnitude is stable against theoretical uncertainties. Our estimate gives ΔΓ Γ of the order of 0.15, a value that could be measured at LEP.