  The existence of instantonic decay modes would indicate a semi-classical
instability of the vacua of ten and eleven dimensional supergravity theories.
Decay modes whose spin structures are incompatible with those of supersymmetric
vacua have previously been constructed, and we present generalisations
including those involving non trivial dilaton and antisymmetric tensor fields.
We then show that the requirement that any instanton describing supersymmetric
vacuum decay should admit both a zero momentum hypersurface from which we
describe the subsequent Lorentzian evolution and a spin structure at infinity
compatible with the putative vacuum excludes all such decay modes, except those
with unphysical energy momentum tensors which violate the dominant energy
condition.
