The interaction between a natural Na+-montmorillonite and a crude oil sample containing 2.5 wt % of asphaltene and 6.4 wt % of polar species is studied as a function of time. The microscopic and the macroscopic mechanisms of diffusion are examined and discussed. At a microscopic scale, diffusion of crude oil within the clay interlayer space is a slow and a continuous process with the formation of O/W microemulsions within the d(001) space, which involves those crude oil polar molecules with tensoactive properties, interlayer water molecules, and the clay Na+ interlayer cations. At a macroscopic scale, the diffusion of the organic species is also a slow but stepped process. At the beginning, such a process is controlled by a concentration gradient (the Fick law) of crude oil species which diffuses into clay pellet, and it is followed by a viscous flow-type diffusion, when the compacted clay mineral pellet is surrounded by a thick O/W emulsion of crude oil, clay colloids, and water expulsed from the interlayer space.