  The radiative decays $\eta'(\eta)\rta\gamma\gamma$ are discussed. The
modifications of the conventional PCAC formulae due to the gluonic contribution
to the flavour singlet axial anomaly are given. The decay constants satisfy a
modified Dashen formula which generalises the Witten--Veneziano formula for the
mass of the $\eta'$. It is shown how the topological susceptibility in QCD with
massive, dynamical quarks may be extracted from measurements of
$\eta'(\eta)\rta\gamma\gamma$.
