A multi-technique approach was adopted using UV/visible spectroscopy, fluorescence spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM) in the characterization of cadmium sulfide (CdS) nanoparticles. CdS nanoparticle sizes could be determined by the Brus equation, which relates the UV/vis spectroscopic properties of CdS nanoparticles to size based on the quantum confinement effect. The diameters calculated from the wavelength of absorbance in UV/vis spectra were within 10% of the mean nanoparticle diameter measured in TEM. UV/vis spectroscopy provided an aqueous phase measurement of the CdS core size of the nanoparticles (<2 nm to ∼10 nm diameter) that is based on a physical property rather than light scattering. CdS nanoparticles readily formed upon addition of sulfide to a Cd(II)–thiolate complex with the thiolate molecules acting as capping agents that passivate the nanoparticle surface, thereby preventing bulk mineral precipitation. Using UV/vis as a method of nanoparticle sizing, we were able to demonstrate how aqueous conditions dictate the resulting nanoparticle size. High pH, capping thiolate concentration and Cd : S ratio all resulted in smaller nanoparticles. Ionic strength did not influence nanoparticle size, but DLS data indicate the formation of aggregates above ionic strengths of 0.1 M.