Measurements of the attenuations produced by air-filled rectangular waveguides of various materials have been made at frequencies of 35, 70 and 140Gc/s. Some effects of corrosion, annealing waveguides in a hydrogen atmosphere, acid etching of internal surfaces and electroplating have been determined. Internal surface finishes of many waveguide samples have been observed and d.c. resistivities of the materials have been measured.Measured losses are all well in excess of the theoretical values as predicted by formulae for perfectly smooth walls (e.g. up to 2.5 times in the case of copper at frequencies of 70 and 140 Gc/s). The discrepancies between calculated and experimental values increase with frequency and cannot be wholly accounted for at millimetre wavelengths by surface roughness. Attenuations can be reduced considerably by annealing or etching waveguide surfaces.Losses in waveguides increase after exposure to the atmosphere, and the corrosion characteristics of tellurium-copper, which has been recommended13for millimetre wavelengths, appear to be no better than those of pure copper or standard silver.Electroplating of certain waveguide components for millimetre wavelengths may be worth while.