The thermoelectric powers and dc and 200‐mc resistivities of pure and metal‐doped selenium have been studied as functions of temperature. The properties of materials in the two important microstructures (colony and equiaxed) were distinguished. The influences of the metals Na, Cu, Ag, Mg, Zn, Cd, Hg, Ga, In, Tl, Pb, Sb, Bi, Te, Fe, Ni, Co, and Ce were noted.A working model of the selenium semiconductor was developed. In this model there is an acceptor level <0.15 ev from the full band caused by nonmetals and possibly lattice defects. In strained crystals or material in the colony structure a second acceptor level 0.27 ev from the full band is postulated from the behavior of dc and ac resistivity curves as a function of temperature. The density of the first level varies in the range 1014/cm3to 6×1016/cm3with amount of nonmetal impurity. The density of the second level depends on the crystal structure, decreasing as the colony structure recrystallizes to the equiaxed. In the specimens crystallized at 110°C the density was about 2×1016/cm3. Both metals and non‐metals are largely segregated at grain boundaries. Metals produce effective donor levels at these places.The dc properties reflect the presence of internal barriers produced by the segregation. The donor levels enhance barriers existing in the purest selenium tested increasing resistivities and thermoelectric powers.With increase in temperature >160°C the density of acceptor levels decreases as defects are removed by structural changes in the lattice.