  Neutrinos that decay leave their imprint on the cosmic microwave background.
We calculate the CMB anisotropy for the full range of decaying neutrino
parameter space, and investigate the ability of future experiments like MAP and
Planck to probe decaying neutrino physics. MAP and Planck should be able to
distinguish from $\Lambda$CDM, eV-scale neutrinos that decay after
recombination. Neutrinos that decay before recombination are distinguishable if
the decays occur very much out of equilibrium; Planck should be sensitive to
the border between equilibrium and out of equilibrium decays. Adding decaying
neutrino parameters to the set of cosmic parameters, we calculate the
uncertainty in the full set of parameters. The ability to measure neutrino
parameters depends sensitively on the decaying neutrino model. Adding neutrino
parameters degrades the sensitivity to non-neutrino parameters; the relative
amount of sensitivity degradation depends on the decaying neutrino model, but
tends to decrease with increasing experimental sensitivity.
