Order-disorder phenomena and structural transformations in substitutional alloys are of great interest in both the theoretical and technological fields of phase stability in general. In this paper, we first briefly review the advanced methodology which has been developed recently to study phase formation. Then we present our results on the effect of a tetragonal distortion of the b.c.c. crystalline structure onto the thermodynamic properties of transition metal alloys. Finally, we discuss our recent predictions pertaining to the possible existence of new ordered states based upon the A15 crystalline structure.