Two tensile-strained GaxIn1-xP / InP (001) heterostructures (with x = 6.5% and x = 11.2%) have been studied by TEM and cathodoluminescence. In both structures stacking faults and twins, located in the epilayer, have been found preferably along the [(1) over bar 10] direction whereas perfect misfit dislocations have been observed at the interface in the [110] and [(1) over bar 10] directions. The two types of structures differ about the nature of the misfit dislocations and about the thickness of the twins and this is thought to be due to the different stress relaxation amounts in these structures.