The transmission in the terahertz (THz) region (up to 3000 μm) of alkali halide monocrystals of sodium chloride (NaCl), potassium chloride (KCl), potassium bromide (KBr) and rubidium iodide (RbI), widely used in the infrared region, was studied. The dependences of the absorption coefficient of these materials in the range 0.9 – 3000 μm were obtained. Near 1000 – 3000 μm, the studied materials are transparent, which allows them to be used in THz devices of the millimeter range.