Phase stability of pseudo-ternary (Ga,ln,Mn)As alloys with a zincblende structure is investigated by means of the full-potential linearized augmented plane-wave method. Calculated results predict that the ordered and disordered states are energetically less favorable with respect to the phase separation. When the formation energy is decomposed into elastic and chemical energy contributions, the alloys with more than 0.2 In composition have a chemical tendency of mixing, while the phase separation is driven by the large elastic energy contribution. (C) 2010 Elsevier B.V. All rights reserved.