±-threo-2-deuterio-1,2-diphenylethyl) have been pyrolysed to elucidate the substituenteffects on the rate and stereospecificity of the reaction. Activation parameters (log A and Ea) for the first eight compounds listed were determined. The reaction was accelerated by α-substitution; this was a combination of both polar and steric effects. The stereospecificity of the reaction was high.