The basis for an equivalent-circuit representation of obstacles and discontinuities of a general type was established for multimode wave guides. The scattering matrix was proved to be symmetrical in the general case which includes dissipative types of discontinuities in structures such as slots in the wall. An application was made to a lambda2 rectangular slot in the wall of a rectangular wave guide propagating TE10 and TE20 modes. The symmetry of the scattering coefficients was in good agreement with theoretical predictions in which the excitation distribution was assumed to be determined by the slot and not by the exciting mode. This assumption was also substantiated by observed radiation patterns.