The current trend towards integrating CMOS circuitry and photonic devices on silicon-on-insulator (SOI) wafers into monolithic optoelectronic circuits requires modification of the traditional rib waveguide to provide a planar surface. One possible modification involves creating a trench on either side of the rib and then filling this trench with a planarizing oxide. The resulting planar surface is much more compatible with the photolithographic systems required to print the small dimensions found in state-of-the-art CMOS. We report the fabrication of prototype trench-isolated waveguides, measurement of optical performance and comparison with simulation. 2.5 μm thick Si film was utilized with 0.5 μm deep trenches ranging from 1 to several microns in width. Rib widths ranged from 2 μm to 3 μm (the maximum value providing single mode propagation). Allowed modes were determined with FEMLAB, while beam propagation was studied using Optiwave BPM. Simulation indicated mode confinement would be lost for trench widths less than 1.5 μm. The narrowest trench width which could be fabricated was 2.0 μm, and qualitative optical testing shows good mode confinement to the central rib for a trench geometry greater than 3.0 μm.