We grew In1−xGaxAsyP1−y/InP quantum wells (QWs) by low‐pressure metalorganic vapor phase epitaxy. The In1−xGaxAsyP1−ylayer was closely lattice matched to InP with a composition ofy=0.9 (x=0.47y). We investigated structural imperfections such as composition fluctuations, interface roughness, and nonperiodicity analyzing the low‐temperature photoluminescence linewidth. We found that the InGaAsP layer composition fluctuated, causing about 5 meV inhomogeneity in the exciton energy level in QWs wider than about 3 nm. Since we obtained very smooth interfaces with less than one monolayer of fluctuation and excellent periodicity by lowering growth temperature to 570 °C, the inhomogeneity of the exciton energy level could be held at 6 meV for 20‐period 10‐nm multiple QWs. As a result, despite composition fluctuations, a clear room‐temperature exciton optical absorption peak was observed at 1.5 &mgr;m for the first time to our knowledge.