D_s K > The analysis of tagged $B_s\to J/\psi \phi$ decays determines
the CP phase $\phi_s$ in \bbms\ with a two–fold ambiguity. The solutions
differ in the sign of $\cos \phi_s$ which equals the sign of the width
difference $\dg_s$ among the two $B_s$ mass eigenstates. We point out
that this ambiguity can be removed with the help of $B_s \to D_s K$
decays. We compare untagged and tagged strategies and find the tagged
analysis more promising. The removal of the sign ambiguity in $\dg_s$
can be done with relatively low statistics and could therefore
be a target for the early stage of $B_s \to D_s K$ studies.