The gas-phase equilibrium acidity of 1H-undecafluorobicyclo[2.2.1]heptane (I), 1H,4H-decafluorobicyclo[2.2.1]heptane (II), and tris(trifluoromethyl)methane (IV) has been determined by pulsed FT-ICR technique. Also, a comparison study of the thermodynamic acidities of I and IV in DMSO solution has been made. These experiments have long been considered crucial for the evaluation of the role of the anionic (fluorine) hyperconjugation on the stability of aliphatic fluorosubstituted carbanions. Substituent field/inductive, polarizability, and steric effects for I, II, and IV can be assumed to be similar. The significantly greater observed acidity of the open-chain IV, compared to its bridgehead counterparts I and II, can be reasoned to be the inability of the latter acids to form planar anionic structures (Bredt’s rule) that is a requirement for maximum hyperconjugation. Both the new gas-phase and the DMSO measurements show that IV is by many powers of 10 more acidic than I and II. Together with the existing experimental and the theoretical work, important evidence is provided that qualitatively favors the concept of anionic hyperconjugation.