In strip-loaded slab waveguides, mode coupling between the transverse magnetic (TM11) and the transverse electric (TE21) modes can occur even if isotropic materials such as InP or GaAs are used. This coupling leads to new eigenmodes with a strong hybrid character, that is, the strength of the transverse side components is in the range of the main components. We show that the effect can be explained by a perturbation analysis investigating a raised strip waveguide, which is thought as a superposition of a buried strip waveguide and a dielectric perturbation in the region of the substrate. The described mode-coupling effect should be taken into account when designing integrated optic components, where strip and strip-loaded slab waveguides are commonly used. On the other hand, the hybrid modes can be used to develop new TE/TM converters. We present a new device based on InP which converts an input TE polarization into a mixture of TE and TM polarization, making it an interesting candidate for coherent receiver chips.