Electron impact dissociative excitation of molecules is used to measure the lifetime of metastable species which cannot easily be produced in the laboratory by direct excitation. Two illustrative examples are given. The lifetime of the N₂(a¹ Pi _g) state is measured to be 120+or-2 mu s and the lifetime of the O(⁵S) state to be 185+or-30 mu s, both in good agreement with earlier measurements. The method described may be extended to many other metastable species of current interest.