  Isospin-breaking in B --> pi pi caused by pi^0-eta-eta' mixing is studied in
a model-independent way using flavor SU(3). Measured branching ratios for B^+
--> pi^+ pi^0, B^+ --> pi^+ eta^(') and B^0 --> pi^0 eta^(') imply an
uncertainty in alpha smaller than 1.4 degree. We find a negligible effect of
pi^0-eta-eta' mixing on alpha in B --> rho pi. Characterizing the effect of
rho^0-omega mixing in B --> rho rho and in B --> rho pi by the two-pion
invariant mass dependence, we point out a way of constraining this effect
experimentally or eliminating it altogether. We show that a model-independent
shift in alpha caused by electroweak penguin amplitudes in B --> pi pi and B
--> rho rho, \Delta alpha_{EWP}=1.5+- 0.3 degree, may be slightly different in
B --> rho pi. Other sources of isospin-breaking in these processes are briefly
discussed.
