The viscosity of a molten electrolyte mixture commonly used in direct coal fuel cells (DCFCs) was evaluated. The measurements were obtained from near the melting temperature to a high temperature at which a considerably bubbly flow was induced by decomposition. A gravity-driven capillary viscometer was employed to obtain the viscosity data under low Reynolds flow conditions, using a modified Poiseuille flow relationship. The importance of carbon dioxide addition in measuring the intrinsic viscosity was clearly observed. In addition, the effect of the bubble suspension on the viscosity was quantified in terms of the volume fraction and capillary number. The results showed that the increase in viscosity was best explained only by the difference in the volume fraction of spherical bubbles in the electrolyte melt.