  After discussing the intrinsic ambiguity in determining the light quark mass
ratio $m_u/m_d$, we reexamine the recent proposal that this ambiguity can be
resolved by applying the QCD multipole expansion for the heavy quarkonium
decays. It is observed that, due to instanton effects, some matrix elements
which have been ignored in previous works can give a significant contribution
to the decay amplitudes, which results in a large uncertainty in the value of
$m_u/m_d$ deduced from quarkonium phenomenology. This uncertainty can be
resolved only by a QCD calculation of some second order coefficients in the
chiral expansion of the decay amplitudes.
