We have measured the effect of high pressure on the scattering from the disordered state of a styrene-isoprene diblock copolymer containing 48 wt % styrene (M(W) similar to 16 500. Small-angle X-ray scattering measurements performed at the Cornell High Energy Synchrotron Source (CHESS) show an increase in the aspect ratio of the scattering maximum and a decrease in the angle at which the maximum occurs with increasing pressure. Within the context of the mean-field theory of block copolymer phase behavior originally developed by Leibler,(1) this behavior is due to an increase in both the radius of gyration of the molecule (R(g)) and the degree of segregation of the system (N-chi). Examination of the data using the theory of Tang and Freed(2) reveals an identical quantitative dependence of R(g) and N-chi on pressure without indicating a role for the free volume fraction used by these authors to treat compressibility effects. We therefore propose that the observed behavior arises primarily from shifts in the structural characteristics of the blocks, rather than from a reduction in free volume. From comparison of the pressure-induced changes in the structural characteristics of the system with the corresponding temperature-induced ones, we predict the pressure dependence of the order-disorder transition temperature in this material, provided that the phase state of this polymer is determined solely by the structural characteristics examined in this experiment.