  Manifestations of strong and electromagnetic axial anomalies in two-photon
decays of $\eta$ and $\eta'$ mesons are studied. Applying dispersive approach
to axial anomaly in the singlet current, we obtain an anomaly sum rule
containing strong and electromagnetic anomaly contributions. The relevant low
energy theorem was generalized to the case of mixed states and used to evaluate
the subtraction constant of the strong anomaly-related form factor $\langle 0
|G\tilde{G} |\gamma\gamma \rangle$. We made a numerical estimation of the
contributions of gluon and electromagnetic anomalies to the two-photon decays
of $\eta$ and $\eta'$ mesons and found significant suppression of the gluon
anomaly contribution.
