The width difference in the Bs--Bs-bar system: towards NNLO

The width difference Delta Gamma among the two mass eigenstates of the Bs-Bs-bar system is measured with a precision of 7%. The theory prediction has a larger uncertainty which mainly stems from unknown perturbative higher-order QCD corrections. I discuss the subset of next-to-next-to-leading order diagrams proportional to alpha_s^2 Nf, where Nf=5 is the number of quark flavours. The results are published in [1]