  We present a microscopic calculation of the breakup cross sections of
$J/\psi$ and $\psi'$ on pions and nucleons as a function of the kinetic energy.
These cross sections are used for the investigation of the $J/\psi$ to
continuum and $\psi'/J/\psi$ ratios in ultrarelativistic heavy ion collisions.
The contribution of produced comoving pions to the $\psi'/J/\psi$ signal is
calculated. While this model can account for the data, the uncertainties in the
parameter values do not allow to exclude the possibility of additional sources
for charmonium absorption, like a resonance gas or the quark gluon plasma.
