Activation volumes Delta V-not equal are presently obtained by measuring the effect of pressure on the reaction rate. It is implicitly assumed that the entire response of rate to pressure is volume related, i.e., acceleration by high pressure reveals shrinkage as the reactants progress to the transition state, and vice versa. However, we now demonstrate that high pressure accelerates some bond-making reactions in an additional, nonvolume-related way, through its elevation of solvent viscosity. Diels-Alder reactions, 1,3-dipolar cycloadditions, and Claisen rearrangements are accelerated by rising viscosity and are therefore subject to viscosity-associated acceleration at raised pressures. This gives rise to the illusion of volume shrinkage along the reaction coordinate toward the transition state, which we term phantom Delta V-not equal. Thus the true Delta V-not equal for these reactions, while negative, is less negative than previously believed. Corrections in Delta V-not equal, calculated from experimental rate-viscosity plots, range up to 61%.