A new approach for performing the WO42- polymerization reaction exclusively inside polyelectrolyte capsules of a micron scale size is demonstrated. This approach is based upon a pH gradient across the capsule shell (2.5 inside the capsule volume and 6.5 outside the capsule) caused by encapsulated poly (styrene sulfonate) molecules. During the first stage of the reaction, different polytungstate anions were synthesized. Crystalline WO3 nanoparticles were formed,inside the capsule as the final, polymerization product.