  The new CLEO and LEP data on the eta-gamma and eta'-gamma transition form
factors have renewed the interest in simple interpolation formulas, valid at
any value of momentum transfer. We are going to show that recent theoretical
and phenomenological results on eta-eta' mixing lead to two-pole forms, where
each pole term resembles the Brodsky/Lepage interpolation formula for the
pi-gamma case and depends on the mixing and decay parameters in a simple
fashion. The parameters, entering the occasionally used one-pole formulas, on
the other hand, cannot be interpreted theoretically in a simple way.
