CuPt B -type ordered GaInP, lattice matched to GaAs, has been grown by metal organic vapor phase epitaxy. Ordered domains and their boundaries are investigated by dark-field transmission electron microscopy. By means of stereo images, we are able to show that domain boundaries are microscopically thin. The fact that they generally appear as broad dark lines in dark-field images is due to the projection onto the image plane and does not mean that there is a disordered alloy surrounding ordered domains. This is in agreement with photoluminescence excitation spectra of ordered samples which do not show any sign of a disordered phase.