While it is well known that high viscosities in the concentration boundary layer have a detrimental effect on mass transfer in ultrafiltration, less attention has been paid to the influence of the bulk fluid viscosity. This paper shows that the increase in the bulk fluid viscosity that occurs as a fluid is concentrated can cause the value of the mass transfer coefficient to either increase or decrease depending upon the flow regime. Under laminar conditions an increase in the bulk viscosity increases the shear stress at the membrane surface which can lead to an improvement in mass transfer. Under turbulent conditions the decrease in turbulence that accompanies an increase in bulk viscosity causes a fall in the value of the turbulent mass transfer coefficient. These two phenomena have been demonstrated from the shape of flux-concentration plots for the polysaccharide gums xanthan and pectin.