  Mixing of the pseudoscalar mesons is discussed in the quark-flavor basis with
the hypothesis that the basis decay constants follow the pattern of particle
state mixing. The divergences of the axial vector currents which embody the
axial vector anomaly, combined with this hypothesis provide a calculational
scheme for the parameters describing the mixing of the pion, eta and eta'
mesons. Phenomenological applications of this mixing scheme are presented with
particular interest focussed on isospin symmetry breaking in QCD estimated as
eta and eta' admixtures to the pion. In contrast to previous work a possible
difference in the basis decay constants f_u and f_d is considered and
consequences of this potentially large effect on the strength of isospin
symmetry breaking is discussed.
