Retinoic acid compositions intended for topical application are incorporated in novel formulations in which they are retained as impregnants inside the pores of porous solid particles or microspheres. The pores form a continuous network open to the exterior of the particles, permitting outward diffusion of the retinoic acid impregnant at a controlled rate depending on the pore size. The impregnated particles are prepared by impregnation of preformed particles with the retinoic acid.