Reactions with dianion nucleophiles were used to determine deuterium kinetic isotope effects (KIE's) in gas-phase E2 and S(N)2 reactions. This approach allows for the direct identification of elimination and substitution products in the reactions of simple alkyl halides. In general, the E2 reactions give large KIE's (similar to 7) for perdeutero substrates; however, elimination isotope effects are attenuated in reactions that occur near the collision-controlled limit. In a very slow elimination, a large KIE (> 20) is observed, presumably due to lifetime effects in the collision complex. The S(N)2 reactions all give secondary deuterium isotope effects near unity, with the exception of one system, where suppression of the E2 channel in the perdeutero substrate leads to an enhancement of the S(N)2 rate and causes a modest inverse isotope effect.