High quality, GaInAs, single quantum wells lattice matched to InP barriers, have been grown by metalorganic vapor phase epitaxy at reduced pressures and with the application of short growth interruptions at both heterointerfaces. The occurrence of multiple peaks in the 25 K cathodoluminescence spectrum of a sample containing nominally two single quantum wells is caused by terraces differing by one monolayer in thickness, ≊ 2.93 Å, in the same quantum well. For their observation as well defined excitonic emission peaks, the terraces must be larger in lateral extent than the free exciton Bohr diameter, namely, ≊ 350 Å. This number sets the lower boundary of the size of the terraces. Cathodoluminescence images were formed for the individual emission peaks and the contrast fluctuations observed with a lateral extent up to ≊ 2 μm give the upper boundary for the extended monolayer‐flat terraces. So far this is the first direct observation of such large growth islands at the heterointerface of the GaInAs/InP system.