The decay channel $\eta^\prime\to\eta\pi\pi$ offers several features of interest: due to
final-state interactions it can be used to constrain $\eta\pi$ scattering. It is also an essential
input for a study of inelastic effects in the decay $\eta^\prime\to3\pi$. In the past, extensions
of chiral perturbation theory have been employed to describe this decay.
In this talk, I will present a dispersive analysis of the decay amplitude that is based on the
fundamental principles of analyticity and unitarity. In this framework the leading
final-state interactions are fully taken into account. Our dispersive representation relies only on
input for the $\pi\pi$ and $\eta\pi$ scattering phase shifts. Isospin symmetry allows us to
describe both the charged and neutral decay channel in terms of the same function.
The dispersion relation contains three subtraction constants that cannot be fixed by unitarity.
We determine these parameters by a fit to Dalitz-plot data from the VES and BES-III
experiments and show that the result fulfills the prediction of a low-energy theorem.
We compare the dispersive fit to variants of chiral perturbation theory.