The applicability of a single, 5-MeV O implant for electrical isolation of epitaxial p+-InP layers on semi-insulating InP substrates has been investigated. For such an implant, the ion range is several times that of the epitaxial layer and, consequently, end-of-range disorder is buried deep within the substrate. Though sheet resistances of ∼5×106 Ω/sq were achieved, irradiation-induced conduction in the substrate limits the maximum sheet resistance attainable. The single, high-energy implant scheme has been compared with a multiple, low-energy implant sequence. For a given level of disorder in the epitaxial layer, the latter yields higher sheet-resistance values though the former offers significant process simplification.