Method and agent for reducing water production from oil and gas wells. The agent comprises polymer, cross-linked particles which are stable in water-free solvents. The particles are manufactured in an oil-in-oil emulsion and are cross-linked with at least one water-stable cross-linker and at least one water-labile cross-linker. The water-labile cross-linker is chosen so that when it is opened or broken in a reaction with water a hydrophilic seat is formed which increases the particle's ability to swell in water. The particles furthermore comprise a controlled amount of an immobilized chemical compound which by means of the reaction between water and reactive groups in the polymer skeleton, become able to form chemical bonds to another particle, to another reactive seat in the same particle or to another compounds thereby increasing the agent's stability in the formation.