This Technical Specification provides guidelines for estimating the diameter and the number concentration of monodisperse cadmium chalcogenide (CdTe, CdSe, CdS) quantum dots (QDs) with a narrow size distribution in a colloidal dispersion using Ultraviolet-visible (UV-Vis) absorption spectroscopy. The analysis of the spheroidal particle size is applicable to the diameter range of 3,5 nm to 9 nm for CdTe, 1 nm to 8 nm for CdSe, and 1 nm to 5,5 nm for CdS and is recommended for samples with narrow size distributions.