  Neutrino propagation in space-time is not constrained to be unitary if very
light states - lighter than the active neutrinos - exist into which neutrinos
may decay. If this is the case, neutrino flavor-change is governed by a handful
of extra mixing and "oscillation" parameters, including new sources of
CP-invariance violation. We compute the transition probabilities in the two-
and three-flavor scenarios and discuss the different phenomenological
consequences of the new physics. These are qualitatively different from other
sources of unitarity violation discussed in the literature.
