A systematic investigation of the structure and local structure of Ni_2–xMn_1+xGa alloys 0 ≤ x ≤ 1 reveals antisite disorder to be a possible reason for higher martensitic transformation temperature of Mn₂NiGa as compared to Ni₂MnGa. The antisite disorder between the Y and Z sublattices of the X₂YZ Heusler structure results in segregation of Mn₃Ga type defect phase and a Ni-rich Heulser phase with a higher e/a ratio. This Ni-rich Heusler phase appears to be responsible for the higher martensitic transformation temperature of the Mn₂NiGa type Heusler alloys.