Phase relations within the system Hg–Pt–Se were studied experimentally in the temperature range 400–800°C using the technique of evacuated silica-glass tubes. In this system, one ternary phase, Pt2HgSe3, jacutingaite, was recently described from the Cauê iron-ore deposit, Minas Gerais, Brazil. At 400°C, the following assemblages are stable: PtSe2 + HgSe + (SeL), PtSe2 + Pt2HgSe3 + HgSe, PtSe2 + Pt2HgSe3 + Pt5Se4, Pt2HgSe3 + Pt5Se4 + (Pt), Pt2HgSe3 + HgPt + (PtHg)ss, Pt2HgSe3 + HgPt + Hg2Pt, Pt2HgSe3 + Hg2Pt + Hg4Pt, Pt2HgSe3 + Hg4Pt + HgSe, and Hg4Pt + HgSe + (Hgv). Below 250°C, the phase HgPt is not stable, and the low-temperature assemblage Pt2HgSe3 + Hg2Pt + (PtHg)ss appears in the system. The amalgams Hg2Pt and Hg4Pt are likely to be found in nature.