Various researchers have used the viscosity ratio to quantify the aging characteristics of asphalt binder. The calculation of this viscosity ratio requires viscosity values of asphalt binder under unaged and aged conditions. The asphalt binder is aged under laboratory conditions using a rolling thin film oven (RTFO) for 85 minutes at 163°C. Because of its failure to capture the aging characteristics completely, researchers have some apprehensions regarding the use of the viscosity ratio to characterize the aging susceptibility of asphalt. Several researchers have reported that activation energy is a fundamental property of a material and can be accurately related to the temporal changes within the material. This article presents a reaction kinetics-based activation energy approach to quantify the changes in the viscosity of asphalt binder when exposed to an elevated temperature. For this purpose, three distinct asphalt binders were aged using RTFO under laboratory conditions for an extended period. The viscosity of unaged and aged binders was measured using a rotational viscometer over a range of temperatures. These viscosity measurements were used to compute the aging index and viscous flow activation energy. The test results and subsequent analysis indicated that the aging index was able to capture short-term aging effects to a limited extent for unmodified asphalt, but it failed with modified asphalt binders. Also, the rankings of binders based on the aging index were inconsistent over the range of temperatures used in this study. On the other hand, the computed activation energy and rate of change in activation energy showed a strong correlation with aging time. Thus, it can be concluded that the RTFO aging process combined with the viscous flow activation energy can be used to quantify the aging characteristics of asphalt binder accurately.