The nonlinear theological behavior of different monomodal and bimodal polystyrene model latices has been investigated. The objective of the present work is to develop a practical approach for the modeling of the evolution of the viscosity of bimodal latices based only on the experimental behavior of related monomodal components. For this purpose, the viscosity curve of monomodal latices is described using the Carreau-Yasuda in order to obtain a phenomenological depiction of the shear thinning behavior. Following this, the Carreau-Yasuda law was extended to describe the complex viscosity behavior of bimodal latices. The parameters of the Carreau-Yasuda model, i.e., the zero shear viscosity eta(0), the high shear viscosity eta(infinity), and the inverse of the onset of shear-thinning tau (tau = 1/gamma(c)) are expressed from the Krieger-Dougherty equation. The extension of the Krieger-Dougherty equation to bimodal latices is based on the porosity model developed by Ouchiyama and Tanaka [Ind. Eng. Chem. Fundam. 23, 490-493 (1984)], and used to calculate the variations of parameters eta(0), eta(infinity), and tau versus the volume fraction of particles for bimodal latices. This modeling approach developed in this manner shows that the complex viscosity behavior of concentrated and bimodal latices can be derived from the theological behavior of monomodal components, which is an extremely useful development from the point of view of process engineering applications. In addition, the phenomenon of shear-thickening behavior of polydisperse concentrated latices is also investigated. (c) 2007 The Society of Rheology.