Threading dislocations in the silicon layer in three different types of the silicon on insulator samples produced by standard and improved separation by implanted oxygen (SIMOX) processes were investigated by synchrotron x‐ray topography, scanning electron microscopy (SEM), and optical microscopy. The densities and Burgers vectors of the dislocations were first determined nondestructively by synchrotron x‐ray topography. Then the line directions of the same dislocations were determined by SEM after chemical Secco etching. Some of these results were compared with results obtained from optical microscopy of Secco etched samples. The threading dislocations in the Si layer were found to occur mainly in pairs with densities of the order of 105cm−2in standard SIMOX samples and of the order of 104cm−2in improved SIMOX samples. These dislocations have an edge character. Other features of these dislocations, such as the distances between two dislocations forming a pair, orientations of these pairs, and dislocations that change their line direction, are also discussed. ©1996 American Institute of Physics.