Abstract The effect of pressure has been measured on the absorption edge of ten heavy-metal halides. In general, a large decrease in the gap is obtained with increasing pressure. The differences between the data on thallous halides and previous data on AgCl and AgBr are explained tentatively in terms of differences in band structure postulated by Seitz and Martienssen. Compounds which crystallize in the same lattice show closely similar shifts with pressure (or density) regardless of the halide involved. HgI 2 returns to the high-temperature form above 13,000 atm at room temperature, as has been previously noted by Bridgman.