In the past few years, due to improved control of the ion implantation process and improved annealing sequences, a qualitative improvement has been realized in the structural quality of SIMOX films. The dense network of oxide precipitates and threading dislocations in the top silicon can be annealed out, reducing the dislocation density from ≈ 1010/cm2 to ≈ 105/cm2 or less1. CMOS transistors and circuits have been successfully fabricated in this material2. However, bipolar devices are sensitive to defect densities in this range, as is VLSI yield. Therefore the defect density must be monitored and reduced. We discuss below some techniques for monitoring dislocations and stacking faults in SIMOX films. Also, a different type of defect, a silicon "pipe" running through the buried oxide has been observed. The origin of these defects, and a technique for detecting them, will be described.