In this thesis, mass transfer of ferricyanide ions through electrolyte nanofluids in a circular tube at laminar and turbulent flow was investigated experimentally. Two different nanoparticcles were used to prepare electrolyte nanofluids. Electrolyte nanofluids consisted of 40 nm ?-Al 2 O 3 and 1-3 nm TiO 2 nanoparticles suspended in ferri-ferrocyanide and aqueous sodium hydroxide as base fluid. Well known electrochemical limiting diffusion current technique was used to measure mass transfer coefficients in fully developed hydrodynamics and under developed mass transfer region. The effect of volume concentration of TiO 2 and ?-Al2O3 nanoparticles and Reynolds number were studied. Diffusion coefficient of dye in distillated water base nanofluids was also investigated experimentally. The effect of nanoparticles concentration and temperature were studied on diffusion coefficients of dye in nanofluids. Measurements showed that mass transfer coefficient increased with concentration of nanofluids up to optimum nanoparticles concentrations and then decreased. The optimum concentration for ?-Al 2 O 3 nanoparticles in laminar and turbulent flow was the same and equal to 0.01% while the optimum concentrations for TiO 2 nanoparticles in laminar and turbulent flow were 0.03% and 0.015%, respectively. The average augmentation for ?-Al 2 O 3 /nanofluids and TiO 2 /nanofluids under laminar flow was about 13.7% and 35.86% and under turbulent flow was about 10.06% and 17.66% compared with the base liquid, respectively. Optimum nanoparticles volume concentration is inversely proportional to nanoparticles size. The concentration boundary layer thickness is well thicker than nanoparticles diameter. Thus, because of micro convection and migration of ultrafine nanoparticles inside the concentration boundary layer, which is well thicker than nanoparticles sizes, boundary layer becomes thinner and mass transfer intensifies. For higher concentrations of nanoparticles, greater than optimum concentrations, the mass transfer coefficient in electrolyte nanofluids decreases with increasing the concentration of nanoparticles. Convective velocity of Brownian nanoparticles depends on average size and type of nanoparticles. This is more affected by smaller and lighter nanoparticles. In the present study, smaller nanoparticles are more effective than bigger nanoparticles on mass transfer. Nanoparticles type, however, has a little effect on mass transfer in ENFs. For mass transfer in nanofluids, nanoparticles Brownian motion and clustering are efficacious. Nanoparticles in laminar flow regime are more effective than turbulent flow regime. The mass transfer enhancement in electrolyte nanofluids was independent of Reynolds numbers. The manner of diffusion coefficients in nanofluids was same as mass transfer coefficients of electrolyte nanofluids with nanoparticles concentrations. The diffusion coefficients of dye in ?-Al 2 O 3 /nanofluids and TiO 2 /nanofluids were 4.06 and 8.42 fold greater than that in distillated water. Optimum concentration in diffusion coefficients experiments was dependent on nanoparticles average size and type and nanofluids temperature. Optimum concentrations increase with increasing the nanofluids temperature. Keywords: Mass Transfer coefficients, Diffusion Coefficients, Electrolyte Nanofluids, Electrochemical Limiting Diffusion Current Technique