An analysis is presented for rectangular waveguides with partial dielectric loading in the H-plane. Accurate results are given for attenuation and phase constant for lossy dielectric, using both exact and perturbation approaches. Particular attention is paid to the abnormal situation where attenuation can be greater for partial loading than for complete loading by the same lossy material. This abnormality was reported by Bui and Gagne but discounted by them as being mathematically spurious. By contrast, in this paper an exact theory has been used and the abnormality is shown to be genuine, and due to the hybrid mode redistribution of fields.