This paper presents a theoretical study of sound propagation in an ocean‐surface duct. It deals with several aspects of the theory from a point of view which has not heretofore been taken in the analyses of this problem. The model used to describe a duct assumes the ocean surface to be smooth and the square of the refractive index to be bilinear. Alternative representations of the sound field excited by a point source are derived, the two playing the most significant role in this paper being the residue series and the normal‐mode representation. It is shown that the depth functions of the residue series do not form a complete set, as those of a normal‐mode representation must, and that the normal‐mode spectrum is continuous, rather than discrete. The completeness properties of the normal‐mode functions are then utilized in a study of the energy‐trapping capabilities of the duct. In this connection, virtual modes are introduced and shown to lead naturally to the derivation of a leakage coefficient characteristic of the exponential leakage of energy out of the duct with increasing range. In addition, a cutoff‐frequency criterion, useful in determining when a surface duct can trap energy, is derived.