We have investigated the optical and structural properties of tensile-strained GaxIn1-xP/InP heterojunctions by cathodoluminescence (CL) in the scanning electron microscope and by transmission electron microscopy (TEM). The lattice mismatch of the samples is ranging from 0.4% (x=5.5%) to 0.84% (x=11.8%). We show, in agreement with previous studies, that the relaxation of tensile-strained epilayers occurs by the emission of partial and perfect dislocations. The numerous twins and stacking faults which are found in the epilayers act as efficient recombination centers for electron-hole pairs and appear as dark line defects (DLDs) in CL images. ''Ladderlike'' configurations of these defects are found both by TEM and CL in samples with a lattice mismatch larger than 0.5%. We also demonstrate that DLDs are contaminated by impurities. Areas with networks of perfect dislocations are found between the DLDs, The analysis of the dislocation types allows us to suggest that the growth of low-mismatched samples is two dimensional, and that it is three dimensional in highly mismatched samples. Finally, the spatial variations of the strain relaxation throughout the samples are studied by 77-K CL spectroscopic measurements and it is shown that these variations can be correlated with the various types of structural defects. (C) 1996 American Institute of Physics.