A description is given of equipment and instruments suitable for the determination of the attenuation of circular waveguides of the type appropriate for long-distance transmission. Essentially the procedure consists of measuring the Q-factor of a resonator formed by the length of the waveguide to be tested (a few meters long), short-circuited at both ends, and deducing the attenuation figure therefrom. Some experimental results are quoted to illustrate the versatility of the equipment, its limitations and accuracy.