Two different water-washed municipal solid waste (MSW) incinerator fly ashes were investigated. The theological behaviour of these fly ashes in aqueous suspensions was studied using a parallel plate rheometer. Shear-thinning behaviour was observed in one sample whereas thixotropic behaviour was found in the other. Viscosity depends on both the volume fraction of solids in the suspension and on the yield stress. The data could be well correlated to a model we proposed which takes into account the volume fraction of solid and the shear rate. The comparison with various models of the literature is discussed. The hydrodynamic theory of particle movement in non-Newtonian fluids is discussed, focusing on inertia and viscous effects. Two modified Reynolds numbers involving respectively the shear rate and the yield stress, and with the shear rate and the volume fraction of solids, are proposed and discussed. The flow regime in the investigated experimental conditions is laminar and governed by hydrodynamic interactions. The results also show that inertia cannot be neglected.