The radiative quantum efficiency (η) of CdSe/ZnS core-shell quantum dots (QDs) embedded in synthetic oral fluid was measured using a thermal lens (TL) technique. TL transient measurements were performed using the mode-mismatched dual-beam (excitation and probe) configuration. Thermal optical characterization of CdSe/ZnS QDs was performed for two different core sizes (3.9 and 5.1 nm) incorporated into synthetic saliva with different potential of hydrogen (pH 4–8) values. The thermal diffusivity (D) and average emission wavelength (⟨λem⟩) results are approximately independent of the pH of the solutions evaluated. The fractions of absorbed energy converted into heat (φ) and η are dependent on both the fluid pH and core size of the CdSe/ZnS core-shell QDs. The dependence on pH was also evidenced by fluorescence measurements, which corroborate the results obtained by the thermal lens technique.