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There are many concerns about the blending between virgin and aged binders in reclaimed asphalt pavement (RAP). Determining the extent of the blending and identifying the main factors affecting the blending between two binders have gained great interest in recent years. Viscosity as a function of temperature, exposure time, and film thickness have been mentioned as the main factors controlling the blending between two binders that are in contact. The in-contact blending between two binders is hypothesized to be governed by a diffusion phenomenon in which material is transferred through a medium because of the Brownian motion of molecules. This study aimed to estimate the rate of diffusion of virgin binder (as the diffusing matter) into the RAP binder, which controls the blending of two binders during exposure. The effect of temperature, which was considered as the main factor influencing the diffusion rate, was evaluated at varying levels. Fick's law calculations and dynamic shear rheometer (DSR) measurements were fitted to estimate the diffusion rate. The results showed a high dependency of diffusion on the temperature (viscosity) of binders. The blending development was also found to be a function of the diffusion rate and exposure time. A DSR test method was applied to RAP mortars (mix of RAP fine particles and fresh binder) to verify the effect of time and temperature on the blending level between fresh and actual RAP binders. The results of mortar testing verified the factors affecting the diffusion phenomenon.