Grain boundaries in bulk polycrystalline InP wafers are preferentially attacked by a 5 HCl:3 HNO3:4 HF etch. Canyons with depths greater than 10 &mgr;m and widths approximately 1 &mgr;m are the most common form of attack. Although the etch has no effect on simple twin boundaries, preferential attack occurs at interfaces formed by multiple twinning events.