Mass diffusion of rhodamine 6G (R6G) in water-based alumina nanofluids is studied by means of total internal reflection fluorescence (TIRF) microscopy. We report a mass diffusivity enhancement that reaches an order of magnitude in a 2 vol % nanofluid when compared to the value in deionized water. Since experiments were performed with positively charged R6G, interfacial complexation between the dye and the nanoparticles was not observed. The effect of local density variations on mass diffusivity measurements is also addressed. An explanation for the enhancement of mass diffusion is presented using arguments based on dispersion, and it is shown that it correctly describes the order of magnitude differences between the thermal conductivity and mass diffusivity enhancements reported in the literature.