Ultrathin InGaAs/InP single quantum well structures, grown by chloride transport vapor levitation epitaxy, have been investigated by low‐temperature photoluminescence (PL). Well‐resolved multiple peaks are observed in the PL spectra, instead of an expected single peak. We attribute this to monolayer (a0/2=2.93 A˚) variations in quantum well (QW) thickness. Separate peak positions for QW thicknesses corresponding to 2–6 monolayers have been determined, providing an unambiguous thickness calibration for spectral shifts due to quantum confinement. The PL peak corresponding to two monolayers occurs at 1.314 eV, corresponding to an energy shift of 524 meV. Experimental data agree very well with a simple effective mass theory.