  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.
