Rock phosphate fractions were examined for their porous structure by nitrogen adsorption. The pore size distribution and the shape of pores does not seem to depend on the particle size. The specific surface area values present two regions, one above and one below that of the particles with 125 mu m diameter. Particles of a given size were leached with dilute phosphoric acid (1.5 mass% P2O5) at 25 degrees C. It was observed that there is a widening of the initial pores during leaching. The particles collected at the initial period of the reaction are pitted and those collected at the final period are disintegrated.