A study on a diblock copolymer melt that can form certain specific interactions between dissimilar monomers is performed first with a mean-field approach and then with a fluctuation correction approach. Flory's interaction parameter X possesses both enthalpic and entropic contributions because of the specific interactions. It is found that not only a lower critical ordering transition but also an immiscibility loop with an upper critical ordering transition can be developed in the copolymer by the presence of the specific interactions and the entropic component in X. The mean-field loop phase diagram is shown to feature a typical sequence of microphase transitions upon both heating and cooling with two continuous transition points at a symmetric composition. It is revealed that the fluctuation effects remove both continuous transition points to significantly shrink the loop. The pressure effects on the phase behavior of the copolymer are also discussed. (C) 2003 Wiley Periodicals, Inc.