The capacity to tailor as wanted the fluorescence’s properties of a fluorophore increases the number of applications were the same fluorophore can be useful, like in imageology. One way to modify these properties is the presence of plasmonic fields nearby the fluorophore, and their origin can be the surface plasmons generated in metallic nanoparticles, like silver and gold, when these are excited. Usually fluorescence quantum yield is studied by conventional fluorescence spectroscopy techniques, but these are subjected to errors from reflection or refraction from the sample and a way to avoid these errors is to use indirect measurements techniques as in the case of thermal lens spectroscopy, which measures the change generated by the sample’s absorption of radiation, instead of measuring the absorption per se as regular spectroscopic methods. This technique is based in the photoinduced refraction index’s change. In this work we studied the effect that silver nanoparticles had in the fluorescence’s properties of ethanolic solutions of rhodamine B, specially its quantum yield, using a mode-mismatched thermal lens setup. We found that the presence of silver nanoparticles lowers the dye’s quantum yield between 4% and 38% which depends on the dye and nanoparticles’ concentrations. The thermal diffusivity’s values showed that the silver nanoparticles are increasing the non-radiant decay velocity of the rhodamine b, which is the reason why the quantum yield gets lower. These results not only gave us information about the studied samples, but also validate the capacity of a mode-mismatched thermal lens system to study fluorescence properties.