A stable E1(cb) intermediate, beta-cyanoethyl anion (1 beta), has been synthesized in the gas phase at room temperature under thermal conditions via the fluoride-induced desilylation of 3-(trimethylsilyl)propionitrile. The reactivity and thermodynamic properties of this ion are reported. The cyano group is found to lower the proton affinity of 1 beta by 29 +/- 6 kcal/mol, which represents a particularly large substituent effect. High-level ab initio and density functional calculations have been carried out on 1 beta and several related species. The computational results are compared to each other, and their accuracy is evaluated.