<p>The additive law of logarithmic viscosity of molten alloys is widely established. However, the logarithmic viscosity in the molten Al–Cu system deviates significantly from the additive law. In this system, the molar volume deviates significantly and negatively at intermediate compositions. Al–Ti system has the same characteristics; however, there are few reports on the viscosity of molten Al–Ti alloys. In this study, the viscosities of molten Al–0, 10, 20, 30, 40, 55&#xa0;mol% Ti alloys were measured using the oscillating crucible method. The measured viscosities of all alloys exhibited good consistency in the heating and cooling processes, and the logarithmic viscosities showed good Arrhenius-type linearity. The viscosity increased significantly with increasing Ti concentration. The logarithmic viscosity of the alloys increased drastically with increasing Ti concentration, deviating considerably from the additive law in a similar manner to Al–Cu melts; notably, the degree of deviation was far greater. The activation energy exhibited a composition dependence similar to that of the logarithmic viscosity. However, the maximum value was observed on the Al-rich side. The activation energy in the Al–Ti system was significantly higher than that in the Al–Cu system, indicating that the attractive force between the Al and Ti atoms was significantly stronger than that between Al and Cu.</p>