Abstract: The invention relates to a process for the preparation of microcapsules of organo-phosphoric acid derivative as capsule core. The derivative has a partial mutual solubility with water. The main feature of the invention is the provision of an inner provisional solid capsule shell of a high molecular hydrophobic substance, such as polymethyl methacrylate. This substance is liable to form a shell on each of the droplets by sedimenting under aqueous condition, while it is capable of dissolving into the capsule core substance, when subject to non-aqueous condition which is established after formation of a permanent outer shell on the provisional shell of each capsule. The provisional shell-forming substance is polymethyl methacrylate, polystyrene, polyvinyl chloride or polyvinyl acetate.