Molecular diffusivity in polymeric systems is often the determining factor in performance and efficiency of devices and systems and thus is one of the most important dynamic properties. In polymeric systems, various penetrants, including gas, solvent, and solute molecules can diffuse, and therefore, a general model that can broadly provide their molecular diffusivity could strongly accelerate the designing of polymeric devices. On the other hand, from the microscopic viewpoint, the molecular diffusive motion can be regarded as a random walk movement subject to an enormous number of molecular collisions with neighboring molecules. Therefore, in the present study, both of the microscopic descriptions common for all penetrants in polymeric systems have been unified into one model. The resultant model is the first general diffusion model that can describe diffusivity of all penetrants, including gas, solvent, and solute molecules in rubbery polymeric systems. The applicability of the microscopic model is demonstrated to be acceptable, except in pure polymer near its glass transition temperature.