  The claim that the light quark mass ratio (m_d - m_u)/m_s can be extracted
from the decay width ratio Gamma(eta' -> pi^0 pi^+ pi^-)/Gamma(eta' -> eta pi^+
pi^-) is critically investigated within a U(3) chiral unitary framework. The
influence of the recent VES data on the eta' -> eta pi^+ pi^- decay is also
discussed.
