The influence of changing viscosity by adding polyethylene glycol (PEG) during microfiltration of silica suspensions in the presence and absence of Dean vortices was determined A new microfiltration membrane module design containing helically wound hollow fibers (with Dean vortices) was compared with a standard commercial cross-flow module (without Dean vortices) containing linear hollow fibers during filtration of poly(ethylene) glycol solutions and silica suspensions. The influence of solution viscosity on permeation flux behavior was evident through two separate effects. First, increased viscosity effected the formation and stability of vortices and hence, wall shear rate and convective back-flow. Second permeation flux was reduced with increasing fluid viscosity. Flux improvements (linear us. helical membranes) of up to 45% were observed for all value of the silica suspensions with and without PEG. The energy required to obtain these improvements, however was considerable. Flux advantages of the helical design decreased and eventually disappear-ed as the viscosity of the solution increased Lip to 12 times that of water at 27 degrees C. New mass-transfer correlations for microfiltration of poly(ethylene) glycol solutions containing silica concentrations for laminar flow in a helical and a linear module were obtained with respect to the solution viscosity, for a silica particle concentration of 0.1 wt. %.