Sludge rheology plays an important role in the design and optimization of anaerobic digesters. Organic matter such as primary and secondary sludge or a mixture of the two sludges enters the digesters for further digestion and stabilization. However, there is little information available on how the rheology of the mixed sludge changes. This paper investigates how the rheology of mixed primary and secondary sludge changes when the volume fraction of secondary sludge is altered. This will help predict the rheology of mixed sludge which is required for the design and optimization of pumping and mixing systems. Mixtures of primary and secondary sludge between 2.5 and 7%TS behave as non-Newtonian, shear thinning, yield stress materials whereby the apparent viscosity and yield stress of the mixed sludge depends on the volume fraction of secondary sludge and total solids concentration. The apparent viscosity of primary-secondary sludge mixtures (with same total solids concentration) increases with increasing secondary sludge volume fraction. This suggests that the weak flocs of primary sludge collapse such that the colloidal like particles of primary sludge become trapped and entangled in the gel-like network structure of secondary sludge. However, when dilute primary sludge is mixed with concentrated secondary sludge (and vice-versa), the apparent viscosity and yield stress of the primary secondary sludge mixture increases with increasing volume fraction of the concentrated sludge regardless of sludge type. This is due to the strengthening of hydrodynamic and non-hydrodynamic interactions within concentrated sludge. A master curve was developed to predict the flow behaviour of sludge mixtures. Consequently, correlations were developed to predict the apparent viscosity and a yield stress of sludge mixtures as a function of volume fraction and total solids concentration. (C) 2015 Elsevier B.V. All rights reserved.