The optical and magneto‐optical properties of weakly guiding single‐mode buried channel waveguides in substituted yttrium iron garnet have been investigated experimentally. The propagation constant difference &Dgr;&bgr; of the two coupled TE and TM modes, which is of particular interest for waveguide isolator application, is found to be strongly dependent on waveguide geometry. Measured results can be explained by the specific growth of the top cladding layer, which occurs into different crystallographic directions around the etched waveguide core. This leads to a local variation of the refractive index, which effects the guidance of light, and to a local variation of birefringence, which effects &Dgr;&bgr;. The measured variation of &Dgr;&bgr; is shown to be primarily caused by growth induced birefringence. It is demonstrated that &Dgr;&bgr; can be controlled to small values at selectable waveguide widths as required for waveguide isolator application simply by an appropriate setting of fabrication parameters.