It has been found possible to measure, with accuracy the conductances, under pressure of up to 3000 bars, not only of regular aqueous solutions, but also of solutions in non-aqueous solvents, where the ionic concentrations are low and the conductances are quite small. Using specially designed apparatus, and standard conductance techniques, two different aspects of the effect of pressure, on the reactivity of substituted organic compounds, have been examined. The dissociation constants, of a series of seven benzoic acids substituted in positions meta and para to the carboxyl group, and of nine similarly substituted phenylacetic acids, have been determined in water at 25σ and at pressures ranging from 1 to 3000 bars. The disassociation constants were found to increase with pressure in all cases. For each series the Hammett reaction constant has been found to decrease with pressure, a result which is in accord with prediction. It has also been shown that for a given reaction to which the Hammett equation applies, a plot of the volume change (ρ”V) at any given pressure against ρ, the Hammett substituent constant, will be linear and its slope will be finite provided that ρ is dependent on pressure; when ρis independent of pressure the plot will again be linear but its slope will be zero. These conclusions are readily derived, and they are supported by the experimental results reported. The rates of the reactions of piperidine with a series of eleven derivatives of bromobenzene, in which the substituents are so placed that steric interactions between the substituents are significant, have been determined in isopropyl alcohol at 75σ over a pressure range of 1 to 3000 bars. From the results at atmospheric pressure, it is clear that the nitro group, which must be planar with the benzne ring to exhibit full mesomeric electron-withdrawal from the reaction site, is forced out of the plane of the benzene ring to exhibit full mesomeric electron-withdrawal from the reaction site, is forced out of the place of the benzene ring by methyl and methoxy groups positioned ortho to it. The reactivities under pressure indicate that molecular deformation occurs with pressure and that the nitro group is progressively forced back into the plane of the benzene ring under increasing pressure. There are also indications that steric inhibition of solvation, shown to be insignificant at atmospheric pressure, becomes increasingly important as pressure is increased.