The Mn-Ga equilibrium phase diagram was reinvestigated. The phase analyses were performed both by powder and single crystal X-ray diffraction methods. A laboratory scale method for the removal of oxides from manganese chips was found and an efficient routine procedure for isothermal preparation of single crystals of compounds stable with the melt was developed. The first method was used to purify the manganese used as reactant in the syntheses. The latter procedure was used to investigate the solid-liquid equilibria up to 950°C. The feasibility of bismuth as flux for growth of single crystals of compounds not stable with the melt in the Mn-Ga system was demonstrated. The use of single crystals for the phase analysis was essential to resolve the phase equilibria of compounds with very complicated powder diffraction patterns. Two of the compounds stable with the melt are approximants to decagonal quasicrystals. These are the tetragonal Mn₃Ga₅ and the novel monoclinic Mn₁₂₃Ga_i3y. The previously unknown structures of Mn₃Ga₅ and Mn₁₂₃Ga_i37 were determined from single crystal data. Both have unique structures with large unit cells. A unifying cluster concept showing the close structural relation between Mn₁₂₃Ga₁₃₇ and several other decagonal approximants was developed. The previously unknown unique structure of the most gallium rich compound, MnGa₆, was determined from single crystal X-ray diffraction data. Previously unknown lower stability temperatures of the ß-Mn solid solution and the compound Mn₂Ga₅ were found.