Method for preparing thin films for chemical optical sensors. The invention relates to the development of thin films comprising acrylic polymers on a light-transparent material as optical sensors that contain a luminescent indicator, the light-emitting properties of said indicator changing depending on the concentration of a pre-determined chemical species present in the medium into which the sensor film is inserted. The films are obtained by thermal or photochemical polymerization of acrylic monomers in the presence of a polymerization initiator and of the luminescent indicator that is physically immobilized in the polymer network formed. The resulting thin films can be used for the determination of pollutants in water.