Abstract: Permeable substantially spherical hollow particles of 1-5000 .mu.m in size are formed having an outer shell of a mechanically rigid porous material. Pores of the shell are through-going pores that connect the inside of the particles to surroundings and allow chemical species to traverse the outer shell. To prepare the particles, impermeable hollow particles are treated with an acid or base under reflux conditions to form pores. The outer shell is formed of anhydrous forms of silicon dioxide, metal silicates, metal borosilicates, metal oxides or boric oxide. Metals and metal alloys are not used in forming the outer shell. Particles containing a polymer are formed by immersing the permeable hollow particles in a solution of components that polymerize to form the polymer, allowing the solution to partially fill the particles via the pores and polymerizing the components.