  The formulae describing the radiative decays $\eta'(\eta)\rta\c\c$ in QCD
beyond the chiral limit are derived. The modifications of the conventional PCAC
formulae due to the gluonic contribution to the axial anomaly in the flavour
singlet channel are precisely described. The decay constants are found to
satisfy a modified Dashen formula which generalises the Witten--Veneziano
formula for the mass of the $\eta'$. Combining these results, it is shown how
the topological susceptibility in QCD with massive, dynamical quarks may be
extracted from measurements of $\eta'(\eta)\rta\c\c$.
