We have analyzed the structural and optical properties of GaxIn1−xAs–GaAs strained-layer superlattices (SLS) grown by low-pressure metal-organic vapour-phase epitaxy. Sample uniformity over 2.5 cm × 2.5 cm has been studied by X-ray diffraction and low-temperature photoluminescence. The sample composition and period are uniform in the longitudinal direction (gas-flow direction in the reactor) and in the central portion (1.5 cm) in the transverse direction. On each side, the In composition decreases slightly towards the edges, as shown by an energy shift of the photoluminescence excitonic recombinations. Comparison of experimental and calculated transition energies in a series of samples, taking into account strain and quantization, shows clearly that SLS grown on mismatched buffer layers arc under additional strain. This additional strain is not present when the layer or whole SLS thicknesses exceed a critical value beyond which the mismatch is partially accommodated by misfit dislocations.