Kinetic isotope effects arising from transition state phenomena such as tunneling an sensitive to changes in pressure, whereas those arising from differences in zero point energies in the reactant state are not (Isaacs, N. S, Isotope Effects in Organic Chemistry; Buncel, E., Lee, C. C., Eds.; 1984; Vol. 6, pp 67-105). A new equation is derived which differentiates between the two origins as a function of pressure. Fitting published isotope effects on hydride transfer between chloranil and leucocrystal violet (Isaacs, N. S,; Javaid, K.; Rannala, E. J. Chem. Sec., Perkin Trans. 2 1978, 709-711) to this equation yields Delta V-Q = 36.5 +/- 3.0 mL/mol for the apparent volume difference between the transition states of hydride versus deuteride transfer, k(H)/k(D) = 7.8 +/- 0.1 for the semiclassical isotope effect due to differences in zero point energies, and Q(H)/Q(D) = 1.44 +/- 0.02 for the transition state effect. Thus, tunneling accounts for 33 +/- 1% of the observed deuterium isotope effect at atmospheric pressure. Incorporating the transition state effect into a pair of Bell tunneling correction equations (Bell, R. P. The Tunnel Effect in Chemistry; Chapman and Hall; London and New York, 1980) allows the precise estimation of the reaction frequency as upsilon(H)double dagger = 797 +/- 12 cm(-1) with Q(H) = 1.97 +/- 0.05. Quantifying hydrogen tunneling in this way provides a powerful new tool for probing transition state chemistry.