This invention provides a separation by implanted oxygen (SIMOX) method for forming planar hybrid orientation semiconductor-on-insulator (SOI) substrates having different crystal orientations, thereby making it possible for devices to be fabricated on crystal orientations providing optimal performance. The method includes the steps of selecting a substrate having a base semiconductor layer having a first crystallographic orientation separated by a thin insulating layer from a top semiconductor layer having a second crystallographic orientation; replacing the top semiconductor layer in selected regions with an epitaxially grown semiconductor having the first crystallographic orientation; then using an ion implantation and annealing method to (i) form a buried insulating region within the epitaxially grown semiconductor material, and (ii) thicken the insulating layer underlying the top semiconductor layer, thereby forming a hybrid orientation substrate in which the two semiconductor materials with different crystallographic orientations have substantially the same thickness and are both disposed on a common buried insulator layer. In a variation of this method, an ion implantation and annealing method is instead used to extend an auxiliary buried insulator layer (initially underlying the base semiconductor layer) upwards (i) into the epitaxially grown semiconductor, and (ii) up to the insulating layer underlying the top semiconductor layer.