Analyzing B_s - bar B_s mixing: Non-perturbative contributions to bag parameters from sum rules

We use QCD sum rules to compute matrix elements of the Delta B=2 operators appearing in the heavy-quark expansion of the width difference of the B_s mass eigenstates. Our analysis includes the leading-order operators Q and Q_S, as well as the subleading operators R_2 and R_3, which appear at next-to-leading order in the 1/m_b expansion. We conclude that the violation of the factorization approximation for these matrix elements due to non-perturbative vacuum condensates is as low as 1-2%.