  We analyse B_{d,s} -> K(*)0 K(*)0 modes within the SM, relating them in a
controlled way through SU(3)-flavour symmetry and QCD-improved factorisation.
We propose a set of sum rules for such penguin-mediated decays to constrain
some CKM angles. We determine B_s -> KK branching ratios and CP-asymmetries as
functions of A_{dir}(B_d -> K0 \bar{K}0). Applying the same techniques to
B_{d,s}-> K*0 \bar{K}*0, we outline strategies to determine the B_s mixing
angle.
