A Landau mean-field analysis is performed to study microphase separation upon cooling in a compressible diblock copolymer with the identical compressibility for both blocks. The inhomogeneity of free volume as well as that of the density of each block is analyzed in the weakly microphase separated system. Excess free volume is shown to be located at the interface between microphase domains in order to screen unfavorable energetics between dissimilar blocks. The phase diagram determined for the copolymer is shown to be closely related to that of the incompressible counterpart. In particular, a symmetric copolymer with no compressibility difference between blocks is found to exhibit a critical point. It is revealed that the relevant parameters for the phase behavior of the copolymer are composition and rT chi (app), where r(T) is the chain size and chi (app) an apparent Flory-Huggins interaction parameter that is dependent not only on temperature but also on compressibility. A model-specific form of chi (app) is suggested here to describe in a unified way the effects of temperature, pressure, and chain size on the phase diagram.