A method of compensation for the effects of a longitudinal slot in a rectangular waveguide section used in a standing‐wave meter has been devised and investigated. The slot causes changes of the propagation constant and characteristic impedance of the guide. Compensation of these effects is necessary in the millimeter‐wave region where they become excessive, since the width of the guide is small so that the width of the slot represents an appreciable fraction of the former. The compensation, which is practically frequency independent, is carried out by an extension of the side walls of the slot into the interior of the waveguide forming two internal ridges on both sides of the slot. The field distribution inside the compensated waveguide is computed by conformal mapping and the height of the compensating ridges determined from waveguide and slot dimensions. Numerical results are presented for an example.