We generalized to diblock copolymer systems the lattice cluster theory, which includes the influence on thermodynamic properties of monomer structure, local correlations, and compressibility (often called "equation of state effects"). The thermodynamic properties are expanded in powers of the inverse lattice coordination number and the van der Waals interaction energies. This cluster expansion for the Helmholtz free energy is represented diagrammatically similar to the case of blends, but diblock copolymers introduce new features into the theory due to the presence of correlations in the region of the junction between two blocks. The theory predicts the composition and molecular weight dependence for both the entropic and enthalpic portions of the effective interaction parameter. The influence of monomer structure on thermodynamic properties is illustrated using model calculations for the order-disorder transition temperature as a function of composition and pressure.