  Charmonium spectroscopy provides strong guidelines to effective field
theories, thereby gaining insight into quark confinement and the generation of
hadron masses. Hadronic transitions between charmonium states have been
measured with unprecedented high precision with the BESIII spectrometer at IHEP
Beijing, China. We present systematic studies of isospin-violating decays in
charmonium, such as $\psi' \rightarrow \pi^{0} J/\psi$, $\psi' \rightarrow
\pi^{0} h_{c}$, $e^{+}e^{-} \rightarrow \pi^{0} J/\psi$ at $\sqrt{s}=$ 4.009
GeV, and their improved measurements of their branching fractions.
