The circular-electric-mode hybrid coupler permits electromagnetic energy to be transferred from a circular waveguide, which supports a TE01mode, to a coaxial waveguide also supporting a TE01mode, or vice versa. The extent to which this transference occurs is controlled by a cylindrical-waveguide ‘drift’ region, which supports both a TE01and a TE02mode. The coupling mechanism is closely analogous to that which occurs in the more familiar rectangular-waveguide hybrid, or Riblet, coupler. The paper provides a theoretical description of such a coupler, the results of which are shown to be in agreement with measurement. The theory represents a new application of the moment method.