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The chiral expansion of the low energy processes $\pi^0\to\gamma\gamma$ and$\eta\to\gamma\gamma$ is reconsidered with particular emphasis on the questionof the evaluation of the two low-energy parameters from ${\cal L}^{WZ}_{(6)}$which are involved at chiral order six. It is shown how extensive use ofsum-rules and saturation with resonances as well as constraints from asymptoticQCD effectively determine their values. Predictions for the widths arepresented for both standard and non-standard values of the quark mass ratio$m_s/{\hat m}$. A precise relation is established between the usualphenomenological $\eta-\eta'$ mixing parameters and those of the chiralexpansion. The large size of the chiral correction to the $\eta$ decay can beunderstood on the basis of a simple counting rule: $O(1/N_c)\sim\ O(m_q)$. Itis shown how this counting rule eventually allows one to include the $\eta'$into the effective lagrangian in a consistent and systematic way.Comment: An important sign error was corrected and the paper has somewhat  expande

physical review. d. particles, fields, gravitation, and cosmology/physical review. d. particles and fields

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Chiral sum rules forscrL(6)WZparameters and its application to