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The assembly of Ga alloys with Ni or Ni alloy has been widely developed for various low-temperature applications in recent years. In the constituent Ni-Ga binary system, however, the phase equilibrium with the phase “NiGa₅” and its stability has scarcely been investigated. The present study used the diffusion couple technique combined with SEM-EPMA and XRD analysis to examine the phase stability and the homogeneity range of the phase. The results show that “NiGa₅” is a stable phase in the binary system with little homogeneity range and suggest that the peritectic reaction <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi mathvariant="normal">L</mi><mo>+</mo><msub><mrow><mi>Ni</mi></mrow><mn>3</mn></msub><msub><mrow><mi>Ga</mi></mrow><mn>7</mn></msub><mo>→</mo><msub><mrow><mi>NiGa</mi></mrow><mn>5</mn></msub></mrow></semantics></math></inline-formula> lies between 112.0 and 115.5 °C. This work provides new information for the modification of the Ga-rich low-T region of the Ni-Ga phase diagram.