Colloids composed of ultrafine nanoparticles (~100 nm or smaller) are called nanofluids. Metals, semiconductors, carbon nanotubes and oxide ceramics are certain materials which are dispersed in a host liquid such as water and organic liquids to make the nanofluids. Recently, nanofluids have received growing attention because of their potential to enhance heat transfer. Since some researchers considered Brownian movement of nanoparticles as the responsible factor in the enhancement of heat transfer, investigation of mass transfer enhancement in nanofluids with similar mechanism has been initiated. Investigations on mass transfer in nanofluids can be divided into two main groups. The first group of studies deals with studying diffusion coefficients in nanofluids and the second group of studies focuses on convective mass transfer coefficients in nanofluids. In this thesis, water self-diffusion coefficient and tracer diffusion coefficient of tert-butanol in water-based silica nanofluids were measured by using pulsed field gradient nuclear magnetic resonance (PFG-NMR) method for silica nanoparticle volume fractions ranged from 0.005% to 1% at three different temperatures (15, 25 and 35 °C). Insignificant enhancement (mostly less than 10% increase) in both water self-diffusion coefficient and tracer diffusion coefficient of tert-butanol in nanofluids was observed. Also, the influence of silica nanoparticles on hydrodynamics and mass transfer of a counter-current spray liquid–liquid extraction column was studied experimentally. The chemical system of toluene-acetic acid–water was used, and the drops were toluene-based nanofluids containing 0.0005–0.01 vol% silica nanoparticles. In addition to nanoparticles fraction in nanofluids, the effects of dispersed phase drop sizes were evaluated making use of three different distributors with different hole diameters. The experiments were performed at fixed volumetric flow rates of dispersed and continuous phases and both mass transfer directions. The results showed that silica nanoparticles have no significant influence on the hydrodynamic parameters, while maximum enhancement of 47.4% and 107.5% in overall mass transfer coefficient respectively for mass transfer direction of dispersed to continuous phase and vice versa were achieved for drops generated by the distributor with the largest hole diameter and silica nanoparticle concentration of 0.001 vol%. Moreover, the influence of hydrophobic silica nanoparticles on mass transfer in a membrane-based micro-contactor was studied. The extraction system of toluene?acetone?water was used, and the organic phase was a toluene-based nanofluid containing 0.001 to 0.1 volume percent (vol%) hydrophobic silica nanoparticles. The experiments were performed at various volumetric flow rates of organic and aqueous phases and at mass transfer direction from the organic to the aqueous phase. The results showed that nanoparticles are more effective on mass transfer at low flow rates. Maximum enhancement of about 31% in overall mass transfer coefficient was observed using 0.001 vol% silica nanoparticles. In all experiements, silica nanoparticle concentration of 0.001 vol% was observed as the optimum concentration. At higher and lower nanoparticle concentrations, smaller mass transfer coefficients were observed. Brownian motion of nanoparticles and induced microconvection are dominant in low concentrations of nanoparticles resulting in enhanced mass transfer. Nanoparticle aggregation, reduction in free volume and increase in tortuosity of the diffusion path because of solid nanoparticles presence are responsible for deteriorated mass transfer in higher nanoparticle concentrations. Keywords: Nanofluid, Silica nanoparticles, Mass transfer, PFG-NMR, Spray extraction column, Microextraction