The viscosity of slags containing crystallized solids is always of great interest for materials scientists working in high-temperature slag processing, as it strongly affects slag flow behavior. Different slag characteristics, such as liquid phase composition, concentration of solid phases, and crystal morphology, have different impacts on the slag viscosity. However, the effect of crystal shape and size is hard to demonstrate directly in high-temperature slag experiments, because other slag characteristics may also change. The present work provides experimental results for the viscosity of two slags from the olivine primary phase field, which have slag assemblies with different olivine shapes and sizes, while the other slag characteristics remain the same. The results clearly demonstrate that the crystal morphology has a pronounced effect on the slag viscosity. A detailed analysis of the slag microstructure also provides insight into their Newtonian and non-Newtonian flow behavior. The experimental results were used to evaluate different models of the effective viscosity of classical/archetypal suspensions. Based on this evaluation, we suggest that elongated olivine crystals, which are randomly oriented in the melt at rest and at lower rotation speeds, can become aligned to shear at higher rotation speeds. The Powell model adequately describes such a rheological behavior of slags containing elongated olivine crystals, while the Krieger-Dougherty model is applicable to slags with polyhedral olivine crystals, with the intrinsic viscosity being close to theoretical values obtained for cuboids and prisms of various sizes. (C) 2019 The Society of Rheology.