Temperature dependent synchrotron x-ray powder diffraction, differential scanning calorimetry, and magnetic measurements were performed on Ni 2+x Mn 1−x Ga (x = 0.20 and 0.35) magnetic shape memory alloys. For x = 0.20, though the monoclinic phase is thermodynamically stable, a trace of residual stress can stabilize a tetragonal phase. The residual-stress-induced tetragonal phase transforms to the cubic austenite phase over an unusually large temperature range (348 K ≤ T ≤ 693 K), suggesting extremely slow kinetics of transformation. In contrast to x = 0.20, the thermodynamically stable phase of x = 0.35 is tetragonal and this composition exhibits the usual features of a reversible martensitic transformation. The results suggest that for x = 0.20 the monoclinic and tetragonal phases are nearly degenerate.