Measurements of the attenuations produced by air-filled rectangular waveguides of nickel, mild steel, Mumetal, Radiometal and Rhometal have been made in the frequency range 9000–9675 Mc/s. The permeabilities of the materials have been determined from these measurements and a knowledge of the roughness and resistivity of each waveguide internal surface. The effects of temperature on the h.f. permeabilities have also been studied, and some qualitative results are included on the effect of superimposing a steady magnetic field on the h.f. one.