The Hg–In phase diagram was investigated by X‐ray diffraction up to 55 at.% In. In the range 0–17 at.% In β phase, a primary solid solution of In in Hg was found. There is evidence for a βt phase at 17 at.% In for which a tetragonal b.c. cell (a = b = 3.48, c = 3.310 Å) is proposed. At 50 at.% In the alloy crystallizes with a rhombohedral cell (a0 = 3.008 Å, α = 73°20′). These results correct some previous published phase‐diagrams and are consistent with residual resistivity curves.