Abstract: Upon the application of 3%yeast suspension as a feedstock to an expanded bed of Streamline AC equilibrated with 0.02 mol•L^-1 phosphate buffer (pH 7.0), the transient variations of local bed voidage and particle size distribution were investigated. The experiments were carried out in a customized glass column (26 mm I.D.) modified by side ports. It was exhibited that the introduction of unclarified feedstock with a high viscosity and density induced the upward movement of particle and consequently a change in local bed voidage and particle size distribution. At a given bed height, voidage firstly decreased to a minimum and then increased to a high equilibrated value in voidage. The transient process not only included a fluctuation in particle size distribution but also involved the change in particle concentration at a certain bed height, particularly at the middle and top of the column.