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Phase diagrams of two-dimensional binary alloy with vacancies (ABV) is studied with pairwise interaction " AA , " BB , and " AB between the nearest neighbors. Attention is focussed on the case where the vacancies are annealed and " BA = ", " AA = " BB = 0 . F or vacancy concentrations cv < 0:2, the critical temperature decreases linearly with cv. The vacancies remain randomly distributed within the A and B phase separated regions. For cv > 0:2 the critical temperature no longer shows this linear trend and falls to zero very sharply at cv = 0 : 39. This critical vacancy concentration is slightly lower than the one expected for quenched vacancies, where the percolation threshold of atomic species is 0.59 (cv = 0 : 41). The mobility o f v acancies and the nature of atomic interactions makes it energetically favorable for the vacancies to occupy the boundary layers between A and B phase separated regions, thus making the process of phase separation dicult. This lowers the critical temperature below that of the case where the vacancies are quenched. Salient features of the phase diagramare discussed by comparisonwith previouslystudied similar systems with dierent i n teraction energies.

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