Ultrathin InAs/InP single quantum wells were grown by low-pressure metalorganic vapor phase epitaxy using tertiarybutylphosphine and tertiarybutylarsine, with nominal thicknesses between 2 and 3 ML. Characterization of a large number of samples by high-resolution x-ray diffraction (HR-XRD) and photoluminescence (PL) indicates an average InAs thickness very close to the nominal one. Photoluminescence excitation and absorption spectra of selected samples contain excitonic resonances associated with electron-hole transitions in 2 or 3 ML of InAs buried in InP. Only one set of these resonances appears in a given sample, thereby corroborating HR-XRD and PL measurements. Thus, our measurements indicate an absence of significant As-P intermixing.