QCD sum rules are used to estimate
the flavour SU(3)-symmetry violation
in two-body B decays to pions and kaons.
In the factorizable amplitudes the
SU(3)-violation manifests itself in the ratio of
the decay constants f_K/f_pi
and in the differences between the B→K, B_s→K
and B→pi form factors.
These effects are calculated from the QCD two-point
and light-cone sum rules, respectively, in terms
of the strange quark mass and the ratio of
the strange and nonstrange quark-condensate densities.
Importantly, QCD sum rules predict that
SU(3) breaking in the heavy-to-light form factors
can be substantial and
does not vanish in the heavy-quark mass limit.
Furthermore, we investigate the strange-quark mass dependence of
nonfactorizable effects in the B→K pi decay amplitudes.
Taking into account these effects we estimate the accuracy of
several SU(3)-symmetry relations between charmless B-decay
amplitudes.