A high pressure X-ray powder camera has been constructed. The instrument has been found to be useful for routine X-ray work, using molybdenum radiation, to pressures of approximately 60 kilobars. Previously reported transitions have been observed in silver iodide, potassium iodide, bismuth, and thallium. The high pressure forms and lattice parameters were found to be: AgI—f.c.c. (NaCl type), a0=6.067 A; KI—s.c. (CsCl type), a0 = 4.093 A; Tl—f.c.c. (NaCl type), a0 = 4.778 A; Bi structure not determined. These data confirm previous reports on the high pressure forms of AgI and KI. Data on Tl and Bi are apparently reported for the first time. The high pressure modifications were studied at the following approximate pressures which are not indicative of the point where the transition occurs: AgI—3.3 kilobars, KI—20 kilobars, Bi—28 kilobars, and Tl—60 kilobars.