The imaging cathodoluminescence capability of a scanning electron microscope (SEM) was complemented by etch pit (EP) measurements and secondary ion mass spectroscopy (SIMS) to characterize structures of HgCdTe LPE grown both on CdTe and CdTe/sapphire (PACE-1) (Ref. 1: W. E. Tennant, IEDM Technical Digest 1983, 704) substrates. In addition to providing the source for luminescence, the high-power-density SEM beam was exploited to perform in situ annealing which improved the CL efficiency and aided in the analysis. Selected CL/SEM data are shown for substrates, epilayers and interfaces. CdTe Bridgman grown substrates exhibited defect structures (twins, dislocations, precipitates, subgrain structure) and substantial differences in CL efficiency and response to electron beam (EB) annealing. Cd annealing of Bridgman substrates increases CL efficiency, removes sensitivity to EB annealing, and causes impurity redistribution as measured by SIMS. PACE-1 substrates show higher defect density, an absence of subgrains, and more uniform defect distribution in contrast to CdTe Bridgman substrates. The defect characteristics of the epilayers grown on CdTe substrates were found to depend on Hg content in the epilayer. CdTe epilayers tend to reproduce CdTe substrates. Hg1−xCdxTe (x≂0.93) epilayers have no subgrains, and a much lower defect density (∼mid 104/cm2). A highly dislocated and disturbed interface was observed in Hg1−xCdxTe (x≂0.3)/CdTe (on a chemically angle etched surface) and in Hg1−xCdxTe (x≂0.93)/CdTe (cleaved cross section samples). Improvement in the CL intensity from the epilayer was seen with distance from the interface. Both termination of substrate defects and origin of epilayer defects at the interface was observed.