According to the invention the process for preparing a composition includes a step of percolating a fluid, preferably at a temperature greater than or equal to 30.degree. C., more preferably ranging from 30 to 150.degree. C., very preferably from 40.degree. C. to 120.degree. C., under a pressure of at least 3 bars (3.times.105 Pa) through at least one protective agent for keratin fibers, in solid or pasty form.