  The mass and width of the lowest lying S-wave spin singlet charmonium state,
the eta_c, are measured using a data sample of 1.06x10^8 psi' decays collected
with the BESIII detector at the BEPCII storage ring. We use a model that
incorporates interference between the signal reaction, psi' -> gamma eta_c, and
a non-resonant radiative background to successfully describe the line shape of
the eta_c. We measure the eta_c mass to be 2984.3 +- 0.6 +- 0.6 MeV/c^2 and the
total width to be 32.0 +- 1.2 +- 1.0 MeV, where the first errors are
statistical and the second are systematic.
