Photopyroelectric techniques were introduced for the optical characterization of fluids in order to evaluate the adsorption efficiency of methylene blue in aqueous solution by mesoporous silica nanoparticles (MSNs). For this objective, optical absorption coefficients were measured at a wavelength of 660 nm for MSNs loaded with different amounts of methylene blue. This optical property was determined under three different conditions: dye solution at different concentrations with MSNs, the supernatants of the previous solution without MSNs (after 30 min of adsorption followed by centrifugation), and the MSNs pellet formed after centrifugation and redispersed in water. It was found that these measurements allowed evaluating the adsorption efficiency of the MSNs for methylene blue. Similar experimental procedures could be useful to measure this property of nanoparticles to pigments in general.