It has been shown in our recent Landau analyses for a compressible block copolymer that its phase behavior is largely described by an effective Flory interaction parameter, which is subdivided into the density-dependent dimensionless exchange energy and the contribution from the 1.2- and 2,2-elements of the second-order vertex functions. Here, it is further shown that for the symmetric copolymer in the long chain limit those two vertex functions are directly proportional to disparity in equation-of-state properties between blocks and the isothermal bulk modulus, respectively. It is argued that the relative strength of molecular parameters governing each part of the effective Flory parameter determines whether a given copolymer experiences ordering (barotropic) or disordering (baroplastic) upon pressurization. Comparison of theory with experimental results is also mentioned. (c) 2006 Elsevier Ltd. All rights reserved.