  The discovery of the cusp effect in the decay K+ --> pi+ pi0 pi0 has spurred
the search for other decay channels, where this phenomenon, which is generated
by strong final-state interactions, should also occur. A very promising
candidate is eta' --> eta pi0 pi0. The cusp effect offers an excellent
opportunity to experimentally extract pi pi S-Wave scattering lengths. We adapt
and generalize the non-relativistic effective field theory framework developed
for K --> 3 pi decays to eta' --> eta pi pi. The cusp effect is predicted to
have an effect of more than 8 % on the decay spectrum below the pi+ pi-
threshold. We also show that with our current theoretical information about
eta' --> eta pi pi decays, it is not possible to extract pi eta threshold
parameters.
