Usefully accurate rate constant data are calculated ab initio for the four titled unimolecular reactions. The issue of what levels of electronic structure theory are necessary to obtain near quantitative agreement with the observed rate constants is explored. In particular, wave function-based energy barriers, extrapolated to the approximate complete basis. set limit, are needed. DFT-based methods significantly underestimate the barrier heights. Considering the various sources of theoretical error, the classical 1,2 HX elimination barriers are 62.7 +/-0.4 kcal/mol for chloroethane and 60.7 +/- 0.6 kcal/mol for bromoethane. 1,1 HX elimination is a barrierless process yielding products higher in energy than the CX bond dissociation products.