  We analyze the isospin equilibration properties of neutrinoless nuclear
($npe$) matter in the temperature and density range that is relevant to neutron
star mergers. Our analysis incorporates neutrino-transparency corrections to
the isospin (``beta'') equilibrium condition which become noticeable at
$T\gtrsim 1\,$MeV. We find that the isospin relaxation rate rises rapidly as
temperature rises, and at $T\approx 5\,$MeV it is comparable to the timescale
of the density oscillations that occur immediately after the merger. This
produces a resonant peak in the bulk viscosity at $T\approx 5\,$MeV, which
causes density oscillations to be damped on the timescale of the merger. Our
calculations suggest that isospin relaxation dynamics may also be relevant when
neutrinos are treated more accurately via neutrino transport schemes.
