The solvent-promoted elimination reactions of threo-1-(1-X-ethyl)indene 1-X (X = Cl, Br, I) and the corresponding erythro isomer 2-Br in 25 vol% acetonitrile in water at 55 °C, which are concluded to be of E1cB and E2 types, exhibit leaving-group dependent, non-stereospecific and stereospecific 1,2-elimination, respectively.