High pressure causes biphasic effects on the oxidation of benzyl alcohol by yeast alcohol dehydrogenase as expressed in the kinetic parameter V/K which measures substrate capture. Moderate pressure increases the rate of capture of benzyl alcohol by activating the hydride transfer step. This means that the transition state for hydride transfer has a smaller volume than the free alcohol plus the capturing form of enzyme, with a DeltaV of -39 +/- 1 mL/mol, a value that is relatively large. This is the first physical property of an enzymatic transition state thus characterized, and it offers new possibilities for structure-activity analyses. Pressures of >1.5 kbar decrease the rate of capture of benzyl alcohol by favoring a conformation of the enzyme which binds nicotinamide adenine dinucleotide (NAD+) less tightly. This means that the ground state for tight binding, E-NAD+, has a larger volume than the collision complex, E-NAD+, with a DeltaV of 73 +/- 2 mL/mol. The equilibrium constant of the conformational change Keq is 75 +/- 13 at 1 atm. The effects of pressure on the capture of NAD+ have no activation phase because the conformational change is now being expressed kinetically instead of thermodynamically, together with but in opposition to hydride transfer, causing the effects to cancel. For yeast alcohol dehydrogenase, this conformational change had not been detected previously, but similar conformational changes have been found by spectroscopic means in other dehydrogenases, and some of them are also sensitive to pressure. The opposite signs for the volume change of tighter binding and hydride transfer run contrary to Pauling's hypothesis that substrates are bound more tightly in the transition state than in the Michaelian reactant state.